Hi,
I am trying to grow 4 key lime trees from seed in Sweden.

My question is, are all seeds true to type or only the ones that create multiple sprouts? Out of 15 seeds, only 2 had twin sprouts and I think that the surviving 4 trees are none of them.

Should I restart with new seeds to ensure good quality or keep going with ones I have.

Many thanks for help!

Key Lime is commonly seed grown ( not grafted ) in several countries, so is
reliably true to type for that purpose.
There is some variation in seedlings as to fruit size and shape etc, but true off types and out crosses seem very rare, so I think your seedlings should be fine.
Keep them until about 15cm tall, and should easily detect any differences.
Polyembryonic seed can have multiple embryos, but probably not all develop fully in the seed, I guess there is a chance you have some monoembryonic seed, but probably +99% true to type.

Agreed.

Further, seedling key lime trees can begin to fruit from seed quicker than most all citrus varieties. A well grown key lime seedling can fruit in just its second year. - Millet

And I have done this here. Do a search.

Thanks for all the answers and for sharing your knowledge!

I was first sceptical due to the winged petioles (small) that the trees have, but after some google exersise I understand that to be normal.

To have fruit the second year in Stockholm would be a miracle probably, the trees can only be outdoors for 6 months a year due to cold weather, but I will update if that happens.