Does it make sense to say "ihan ihana!" (presumably to mean "absolutely wonderful?)
And if it makes sense, can it be used in conversational situations?

查看翻譯

i'd say that sounds unnatural, even though it's grammatically correct.

"ihan" is kind of making thisgs average, like sayng "ihan ok" or "ihan kiva" meaning it' average. but "ihana" is so powerful feeling that they clash a bit.

I think that the fact that those word are probably originated from one makes it sound odd and one would't use them together.

For the meaning "absolutely wonderful"
I'd use these:

Super kiva, sairaan ihana, sika kiva, just hyvä
ihan mahtava (this sound the most natural way)...

And all these are parts of spoken language or casual written messaging, as a reaction to something.