came up in my sisters garden.
Definitely some Rumex, aka dock or something. There are garden species, but IMO that type is a weed
its a weed in my garden ... do NOT let it seed
I don't see the basal leaves of Rumex/dock.
It looks like a Persicaria to me, probably something along the lines of Lady's Thumb, which is known for the dark "thumb-print" smudges mid-leaf.
In the US, these are mostly weeds with attractive flowers (in this case, probably pink), though in Europe there are garden varieties. And Ken's correct that you don't want to let it go to seed.
It's got the chevron on the leaf that I associate with smartweed/knotweed.
In anycase, I'd pull that sucker
Sorry, got Rumex and Polygonum/ Persicaria mixed up,
Lin, they're all in the Polygonaceae; I should have mentioned that ... which makes the Persicarias and the Rumexes cousins or something....