The biggest problem with silicon sealants is nothing will ever stick to them - even more silicon sealant. Therefore, the the original application of silicon sealant fails, there is not much you can do to fix it. If you put the silicon over some old Dicor sealant, you will probably be OK for a while, but you need to go back and get all the old sealant as well as the new silicon sealant completely off before re-applying a new coat of Dicor, EternaBond, or other proper sealant. They do sell silicon sealant removers, but I can't vouch for how well they work ....
Silicon Removal