It is called,Lychnis..and a good hardy Perennial,as you will know..some call it Rose Campion..

Lychnis coronaria. I've only ever seen pink ones.

I too have both and there is a thrid one that is white with a paler magenta eye. both self seed freely.

I have this in my garden, too - very cheerful and undemanding - it seems to self-seed (though die out in original planting occasionally) and you may get pink (a very bright pink) as well as white and red. I'm never going to get these plant names correct, here in France it's called 'Compagnon rouge' (= 'red companion', OK, I can see the resemblance in name) however, they say the botanical name is given as 'Sile[accent on the e]ne'? Definitely same plant though (at least, I think so!) Any elucidation welcome!

Lychnis coronaria has been self seeding in my garden for 20 years and I get flowers which are white, pale pink, deep pink, and white with pink patches. If deadheaded regularly it will flower for many weeks.

Once upon a time this plant was included in Silene, but then it was moved to Lychnis which is a closely related genus in Caryophyllaceae. The change was a fair while back so people ought to be used to it being Lychnis. Mind Pelargoniums are still called Geraniums and that name was changed about 300 years back so I suppose one will just have to wait.

Mine has never self seeded,JayKaty...probably because I don't give it chance..too tidy with always dead heading ! :o)