Is Slovakian unique in having "long/short consonantal vowels"

"in Czech and Slovakian, there is distinction between long and short vowels"

It's similar in Polish although to a lesser degree, and the vowels have no accents.

"Now I've come across geminate consonants in some languages, which sound longer than single consonants"

There're some good examples in Polish with "nn", "kk", "ss".
In words like Anna, wanna (bath tub), panna (unmarried woman), lekki (light - weight), miękki (soft) you clearly hear the long geminate consonants.

There's also something that's quite unique in Polish, namely geminate consonants at the beginning of a word:
ssać (verb) - to suck, ssanie (noun) - suction
zsunąć (verb) - to push down. Here "zs" is pronounced as "ss"
zzuć (verb) - to take of (shoes)
dżdżysty (adj.) - misty-rainy. "dż" is pronounced almost like English "j" in Jerry.
Actually, I don't know if there are similar examples with geminate consonants starting a word in Slovak or Czech.

I'd also add that in Polish words with geminate consonants the pronunciation is essential, ie. that the two consonants are pronounced clearly. If not, in some cases, they'd have a different meaning.
Examples:
"panna" - unmarried woman vs. "pana" - man (mr, sir) in dative, accusative
"lekki" - light (weight) vs. "leki" - medicine (pills)
"ssanie" - suction vs. "sanie" - sled

Do you mean long vowels as in Japanese, like the O in Osaka, and long consonants as in Japanese, like the TT in kitte?

I think he said what he meant. But I'll try again.

In Czech and Slovakian, and I think in Serbian, r and l can function as vowels. You get words like Krk and Srb.

Apparently in Slovakian, some of those vocalic L's are R's are long and some are short.

OP is asking whether Slovakian is the only language in which vocalc consonants can be distinguished as long and short.

Well, in Japanese the city Osaka is properly pronounced in four beats, with two beats on the O, because it's a long O. Is that what we're discussing?

The confusion -- as should be obvious -- arises because "long" and "short" vowels have a completely different meaning in English, signifying the difference between mope and mop, for example.

No, that's not what we're discussing.

"Now I've come across geminate consonants in some languages, which sound longer than single consonants, they have them in Italian and Arabic among others, but this seems to be a different phenomenon. Or is it? And is it unique to Slovakian?"

But that discussion right here in this thread seems to precisely describe the long consonants in Japanese words like kippu and kitte. Does it not?

Yes, but as OP says, "this" = the subject under discussion, long and short vocalic consonants, in particular r and l, seems to be a different phenomenon.

Do you think it is a different phenomenon? I do.

I don't know if it's unique to Slovakian.

Many thanks to you Vinny for attempting to clarify my point so accurately. The long consonants in Japanese words like kippu are precisely the geminate consonants I was asking if they were really the same. Vinny thinks not.

On Tonieja's side point, you can't have a geminate consonant at the start of a word in Czech, as far as I can see - they seem always to insert an e to prevent it happening where otherwise you might think it could. But I believe there are other languages where you can have them. For example if you read Alexander McCall-Smith's No 1 Ladies Detective Agency, set in Botswana, where the main language is Setswana, we are led to believe a polite address to a woman is Mma, and to a man, Rra. And having seen a TV adaptation, these doubled consonants seemed to be pronounced geminately, rather than being a curious digraph. Not a brilliant source, but it's late.