Double flowered opium poppies. They self seed all over the place and often revert to other colours and types. The seed could have blown in from a neighbour or just be a reversion of the red type you have. Our opium poppies are predominantly purple ones but single flowers, and come up all over our garden if we leave them to seed.

Aren't they gorgeous?

I suspect it is a colour form that due to cross pollination over a few generations [or from a plant nearby] has repeared in the colour mix. If you dont like them then just pul them out. They are very pretty though.

Thanks everyone for your helpful comments. I shall leave them in as I do agree they look lovely!