It tends to prolong the flowering period if you do it when they first start to seed but it won't flower over again - well mine never have.

no mine don't either. but it will stop lots of self sown ones. though you can get some lovely colour combinations.

Mine come up every year without fail.

Yes so do mine. I thought the question was about a re-flowering on the same plant later in the season.

Oh! I thought the question was a bit strange seeing it's a perennial.

I assumed it was also about repeat flowering in the same season.