Agreed do not double the dose, copper products at double the therapeutic dose will certainly kill more sensitive fish and really stress out even the most tolerant fish. When dealing with Ich the last thing you want to add is stress. If you are not using Cupramine already that's what I would try next after "stripping" whatever copper is remaining in your QT either by water changes or by using a product like cuprasorb, and just restart your medicating. One other thing you may want to double check is whether you dosed for saltwater vs. fresh because there is a significant difference in doses.