The genital warts can appear sometimes even one or two decades after the exposure, so, though, one is not sexually active, due to previous exposure (as the virus stays in the body and suddenly become active), they can come anytime. Duofilm solution is not very effective in genital warts and other treatments are necessary. Please read this resource for the treatment;
Please feel free for your follow up questions.
On the foreskin, application of Duofilm should not be done. As a matter of fact, use of any over the counter medicine, should not be done as foreskin is very delicate, I am afraid. This is likely to be after the sexual exposure, but if you feel, that they were there before the sexual exposure, they might not be genital warts. An inspection by a physician can only confirm this.
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Feel free, if you have further queries.