In lineages not directly discended from one another (though they do have a common (unicellular) ancestor). According to the article I linked to it even happened to prokaryotes, though “complex” multicellular organisms (whatever that means -- I guess cells not only bound together but also specialized?) ‘only’ evolved six times among eukaryotes.

It is possible that eukaryotes are particularly propense to becoming multicellular the way the OP claims bilaterians are propense to becoming intelligent, but I'd interpret each item of the poll to be conditional on all of the above, and I'd take “complex single cell life” to mean eukaryotes on Earth or similar.