So I’m fairly comfortable with my whistle playing, but I find that playing the same notes multiple times in a row is very difficult, especially when playing faster tunes. Does anyone have any suggestions on how to maybe make it easier?
Yes… but can you give us an example of the kind of passage that is bothering you? It would be more helpful to make suggestions for a particular phrase rather than simply answer “use cuts” (or rolls, cranns or thingies).
The easiest way to try this, is to use the tonguing technique.
Pick up any basic book - recorder; flute or whistle - and the will show you how to set a rhythm going: for instance, in 4 beats in a bar, you make the ‘T" sound with your tongue, then alternative with ‘D’ (or ‘K’ if you prefer’).
So a 4 beat bar will be 'tongued like: T K T K
You can do 8 semi-crotchets in a bar: TK TK TK TK
Then you can master permutating this: TKK TKK TKK TKK etc.
When you ‘tongue’, the hardest part, is making each repeated note consistent. So you have to learn how to ‘articulate’ the beat properly.
The simplest way for beginners, is to use the roof of your mouth as the limit. So when you have to play 4 fast notes in a sequence - say ‘T K T K’ and hit your tongue against the roof of your mouth.
This will cut the sound dead, and shorten the note, so making it ‘sound’ fast and pip-pip like.
This technique of tonguing, is probably the easiest. There are better methods, which include using your throat and diaphragm, but it is easier to learn how to articulate fast beats by using your tongue and mouth roof as a stopper.
Try it!
If you’re playing Irish music (which I assume you are if you’re on this site), tonguing is actually somewhat “frowned upon”. If you google “whistle” and “cut” or “tap” or “strike”, you’ll find a wealth of info. You should also try “rolls” and “cranns”. A search on C&F should yield results, too.
Pat
Most people on this site probably do play at least some Irish music, but the whistle is used in a variety of other types of music, including Scottish.
Only by people who don’t know any better 
See http://www.rogermillington.com/siamsa/brosteve/tonguing.html
Tonguing can certainly be used, but it varies by situation which is why StevieJ was asking for a specific example.
If, for example, you’ve got a jig with three repeating notes at the end of a phrase, I’d tongue the first two to separate them, and then use a cut to separate the third. That’s just my preference, I personally wouldn’t use cuts to separate all three, but using cuts to do so isn’t “wrong” either.
From the article linked above:
There you go – want to play in an authentic traditional style, much like an “open-style” uillean piper playing whistle? Just stop tonguing, and you’re practically there! Some whistlers today do play in a style a lot like the above clips. I’m not one of them by the way – I tongue quite a lot. But I myself find it good practice to do this no-tonguing exercise, and I should probably do it more often!
I DID say “somewhat” frowned-upon - I don’t think you can play whistle without some tonguing, but you should try to minimize it.
Pat
well one song is the Kesh, it seems there are some repetition of notes, but instead of playing them all, i’ll drag one note out longer and make 3 notes into 2 for example. that’s the main song i have been workng on recently, but second “part” (i don’t really know musical terms) of the song has been giving me difficulty for some time now
The Kesh Jig is a tune, not a ‘song’! 
Songs have lyrics.
The Kesh jig doesn’t have as many repeated notes as some poor notations would have you believe, which once more stresses the importance of listening to the tune you’re about to learn played and played well.
GFG {A}GAB ABA {c}ABd edd gdd edB d would take care of the first bit
{c}BAB dBd ege d for the start of the second part
{a}gfg aga bgg {a}g for the ending
repeated notes as I wrote them that are not separated by a cut are separated not as much by tonguing but by a glottal stop. There isn’t really one blanket reply to your question that will cover all note separations in one go. I didn’t include the option of rolls above, first things first.
MT posted a Kesh tutorial a while ago…
Discussion: https://forums.chiffandfipple.com/t/kesh-rolls-and-cuts/58147/1
MP3 link: http://www.box.net/shared/1t7xdc9jne
Oh, I’d forgotten about that. 
Mr. Gumby also posted a nice beginner-friendly clip of The Kesh here:
http://www.whistlethis.com/index.php?content=YkdsemRHVnU%3D&utid=TVRJd09RPT0%3D
I was told that “tonguing” was the wrong way to play repeated notes on a whistle or on a flute.
And the right way was puting “grace notes” between repeated notes. Something like:
E f E d E
Tonguing works pretty good to me when I play the first part of the Kesh Jig, even it been “wrong” as I was told. And I’m still a noob, so I don’t get too easy what people say about grace notes and ornaments. Is there a video explaing that “slowly”?
Ryan Duns has a video lesson about cuts - that was where I started to get a clue as to what they were. I think those are the “grace notes” you are talking about, though they aren’t the same as grace notes in classical music.
You might need to do Lesson 4 first to learn Star of the County Down (the tune he’s using to teach about cuts).
Nice! I didn’t saw that. I’m already practicing it.
So just to be sure, both tonguing and cuts (like Ryan in the video) are right?
The short answer is “yes,” but it’s really more complicated than that. There are places in tunes where a cut is more appropriate than tonguing because it provides the extra bit of emphasis you need in order to accentuate the beat, and there are places where tonguing or glottal-stopping works better than a cut. For example as I mentioned above, some jigs have three repeated notes at the end of the A part or the B part, and if you separated each one with a cut it might sound a bit repetitive. I usually tongue the first two to separate them and then cut the third note for emphasis. [Edited to add link to sound file] Listen to this example:
https://idisk.me.com//bhurley/Public/jigdemo.mp3
You’ll hear a couple of instances here of three repeated notes; the first two are separated by tonguing and the last repeated note is cut instead.
The best way to learn all this is by listening to good players and paying attention to what they do; after awhile it’ll all sink in and you’ll instinctively know where a cut is needed and where you might be able to use tonguing. And it’s also a matter of personal style. There are whistle players who never tongue and yet manage to play rhythmically (Seamus Ennis was a good example), and others who use tonguing as a form of ornamentation (Sean Ryan or Brian Finnegan being examples in that camp). You can also experiment with glottal-stopping instead of tonguing; many whistle players do that as well (listen to Larry Nugent, for example; and while I haven’t heard definitive confirmation of this there are some people who claim that the late great Denis O’Brien (Donncha O’Briain) used glottal stops and never tongued.
Good advice there from Brad - just a note to add, at the risk of confusing people, that cuts (single grace notes) can be tongued. Most commonly when they fall on a downbeat - and unless I’m mistaken you can hear quite a bit of that in Brad’s clip above.
Also worth pointing out that in addition to the techniques mentioned here (cuts, tonguing, glottal stops) the tap-pat-hit-tip-strike device can also be used to separate repeated notes too.
Oh yeah, that was Week Six. 
He calls it a hit. (Why does this one have five different names, when everyone calls a cut a cut?)
Well, you can call a cut a blip or a hop if you want… not that anyone really does. And no-one has mentioned slam or bounce as tap alternatives… Tap is by far the most common and probably the best description, but really they’re all just descriptions, not “correct” technical terms.
Just to chime-in on this. Unlike the “classical” school of wind-instruments, in ITM/Celtic/Traditional you don’t tongue for articulation. Tonguing can be used as an ornament of sorts or “down-beat emphasis/attack” as mentioned by StevieJ. Articulation on the whistle or simple system flute is to be done with fingers (“Finger articulation”). It adds that little traditional and lively touch to the tune or song you’re playing.
I come from a very classical “training” background. It was really hard for me to stop thinking of tonguing as an articulation. It helped when I started thinking of those who play the pipes - they can’t tongue repeated notes .
I’m still working on finger articulation. I’m just starting to feel a bit comfortable with the cuts on a few notes, yet out of nowhere a few weeks ago I was able to do a roll on E. It takes a lot of time to get finger-articulation down, but it sounds so great when it does compared to just tonguing. I listened to one of my old classical pieces (Bach) not too long ago and couldn’t stand how “lifeless” the flute part felt with just tonguing (even with dynamics).