Useful Finnish phrases:

Keksit keksit. = You invented cookies.
Karmi karmi. = The frame was scary.
Ruokin ruokin. = I fed the auk.
Ruletti ruletti! = Roulette ruled!
Jätti jätti. = The giant left (me).
Riitti riitti. = (I) had enough of the rite.
Hain hain. = I fetched the shark.
Kannan kannan. = I'll carry the canna lily.
Teetä teetä! = Have someone make tea!
Koita koita! = Touch the moth!
Voita voita! = Win butter!

(Edit: "rapsi rapsi" deleted.)

@Jourei @simolaitakari @MerriNet @visapollari
I'm making a webpage that includes some of the phrases you posted. If you want to be credited in some other way than "Jourei", "Simo L.", "Vesa P." and "Visa P.", please tell me.
@0x10f Neat! Jourei is fine.
Haukkaa haukkaa! = Bite the hawk!
Hummaamme hummaamme. = We're drinking away our horse.
Kairaa kairaa! = Drill the wilderness!
Katon katon. = I'll check out the roof/ceiling.
Kolaa kolaa. = Keep scooping up the cola.
Korvaamme korvaamme. = We're replacing our ear.
Kyttää kyttää! = Keep an eye on the cop!
Mollaamme mollaamme. = We're scolding our ragdoll.
Muusaamme muusaamme. = We're mashing our muse.
Törmään törmään! = I'm crashing into the (river) bank!
I collected the phrases here. There are also a couple of violent ones I didn't post in this thread.
https://qalle.neocities.org/finnrepeat
Finnish phrases with repeating words – Qalle’s home page

@0x10f Someone should start a podcast or Youtube channel or blog called "Fun with Finnish".
@0x10f nice! Not exactly the same mold, but I also like the classic Vihdoin vihdoin vihdoin.
@0x10f How does that work? Position determines whether it's a noun or a verb? And the same token has totally different meanings when it's a noun and when it's a verb?
@BenAveling @0x10f mostly inflection, e.g. "hakea" = to fetch, "hain" = I fetched, "hai" = a shark, "hain" = accusative form of "hai". https://en.m.wiktionary.org/wiki/Appendix:Finnish_nominal_forms#Accusative
Appendix:Finnish nominal forms - Wiktionary, the free dictionary

Wiktionary

@BenAveling Also, if a verb in 2nd person singular imperative form is followed by a consonant, the consonant is pronounced long. The last three phrases are pronounced approximately "teetä tteetä", "koita kkoita" and "voita vvoita".

Edit: Looks like there's a similar phenomenon in Italian, as in "andiamo a casa".
https://en.wikipedia.org/wiki/Syntactic_gemination

Syntactic gemination - Wikipedia

@0x10f "Hain hain" is useful in IKEA. 

@Tattie @0x10f

Yes!
I was wondering if "Blåhaj Blåhaj" meant "Go and get a plush apex aquatic predator toy"

@0x10f Another reason to dump Duolingo.
😆
@0x10f Kannan kannan kannan - I carry the canna lily's stump?
@Jourei Correct.

@0x10f @Jourei
This particular one can really get long with all the different meanings "kanta" can have.

Kannan kannan kannan kannan kannan - I carry the canna lily's stump's database's opinion

You can make it even longer but I don't want to do the mental gymnastics required for figuring out all the correct translations.

Bonus ones:

Tuli tuli = the fire arrived
Laki laki = the tip of the law

@0x10f

keitä keitä = boil who?
siitä siitä = inseminate from there.
laita laita = set up the edge

@0x10f

This sounds like a fun vocabulary to go on vacation with.
I know where I'm going next summer :)

@Thelsim @0x10f I'm on the ferry to Helsinki as I'm reading this.
@Thelsim @0x10f Sadly, I was not able to work any of these into casual conversation today. Having enough trouble with please and thank you.

@wiredfool @0x10f

I know absolutely zero Finnish, this will be the only vocabulary I'll get to use.

Who needs "thank you" when you have "Win butter!"?

@0x10f
I like to voita voita!
I like to voita voita!
I like to voita voita!
You like the VOITA!
@sumokirby Sorry, it would be "I like to voittaa voita" and "you like the voi".
@0x10f Nice! Trying so hard not to be that guy and say that one of these is incorrect (because nitpicking is not the point) – not completely succeeding. 😀
@tero Oh, that must be "rapsi rapsi".
@0x10f Nope! Wanna try again? : )
@tero Do you mean "koita koita" should be written "koeta koita"?
@0x10f Yes!
(I know I’m fighting a losing battle trying to keep the verb koettaa alive but sometimes just cannot resist dying on this tiny hill) 😃