Posted: 13/09/2012 at 21:36
This one? - http://www.clematis.hull.ac.uk/new-clemdetail.cfm?dbkey=617
Did you prune it this spring? If so you'll have taken out the flowering wood and it's gone staright to producing the scond flush on this season's wood.
Or it may just have had a hard winter or been badly frosted when forming the double flower buds for the June display or just be too hungry to produce the usual first flush of doubles and has gone straight for the second flush of singles.
Don't feed it any more now and don't prune it either but do make sure it gets a good feed of proper clematis food next spring and see if that does the trick. In future years, prune lightly after the first flush of flowers and feed it again to encourage the second flush in late summer.