It looks like houttuynia flore pleno to me, it spreads by underground rhizomes and can be quite invasive, very pretty though, I expect you know it likes damp soil, hope this solves your mystery. H.
This plant is a bog type plant, It will invade your garden unless you restrain it, i.e- keep in a tub, etc.
In 2 years it has spread into my neighbours garden and into my greenhouse. so BEWARE.