Fiona + Unyielding Spirit + Dawn and Dusk = Three unkillable Fiona's that don't leave after the round and all stack her win condition.

Do something about Unyielding Spirit already! That card is BS and people are finding even more BS ways to use it.

The combo you're describing sounds like a huge pain, and a way to severely abuse the limited weaknesses of Unyielding Spirit, but it also sounds very slow. I would expect an Ionia/Demacia deck featuring Fiora to win well before playing such a slow combo.

That said, are you saying that each Fiora adds to the kill count of the other Fiora's? (That is, the first one gets 1 kill for each, the second gets 2 kills for each and levels, the third gets each to three kills?) If that's the case, that sounds like it should be treated as a bug, as the text on Fiora clearly says "I've killed 2 enemies" and therefore ought to be scoped to the individual unit, not "Fiora's in play" generally. (Note that I'm not saying Riot considers it a bug, just that I think the template implies the opposite and that it risks breaks like this in the balance of various Champions...)

Please share where your 17 mana was spent so we can help you.

That interaction doesn't work that way. I've tried it before. If you have a Fiora that's copied, the kill count that was on that Fiora will also get copied (Say this Fiora killed one only and you copy it and the kill count on the new Fiora will be 1/4). But each Fiora have different counts (If both Fiora are at 1/4 and they both each kill one unit, both will be at 2/4. If one Fiora kills and the other doesn't, then one will be 1/4 and the other 2/4.)

You can play that combo at best in round 6. But a Demacia Ionia Fiora deck is supposed to win already at round 6, independent of such a combo. So you can take one copy of dawn and dusk, but I think more than that will not benefit your deck.