I've found that I sometimes get an extra flower after I've cut the spent blooms off. One year I go one spray of flowers in September.

so not common but yes occasionally

Mine throws up the odd spray of flowers after the first flush too..They do prefer a shady position too,which seems to prolong the flowering,and also stop the foliage yellowing early,since I moved it to a north facing border,in front of a Camellia,a couple of years ago..