www.ukbutterflies.co.uk

It's probably not normal, (it may be for all I know), but is it common?

This Peacock appears to have three antennae, albeit one is shorter than the other two.

is that a hint of another butterfly behind the one pictured

Well there certainly is a wingtip extreme right so presumably several Peacocks on these flowers. I agree: maybe you didn't see one lurking behind the main subject.

Jack

There were certainly a lot of Peacocks nectaring on the flowers, (Hemp Agrimony?), probably about twenty, and one rather feisty Meadow Brown which kept on dislodging the Peacocks.

There is another peacock to the right in the full pic, but it's completely separate from the subject.

Different crop. A second ( or third) butterfly is the obvious answer but I'm struggling to see how there could be one there and I can only see one of its antennae - I can't help feeling there should be some other part of it visible somewhere.

That 'third antenna' clearly appears from behind the eye of the front peacock and is not a part of this butterfly.

I also believe that the other antenna is just visible above the right wing (LHS of pic). There is plenty of room for another butterfly to be hidden by the large plant, possibly tilted upwards, on the far side of the plant.

OK, thanks everyone, I'll bow to the general consensus of another butterfly, (although it still seems odd to be unable to see any other part of it) but I can see what Mike means about where the extra antennae is positioned. Thanks again.