I'm not aware of any danger from melatonin but high doses can cause headaches, nausea, nightmares, and daytime grogginess.
5mg is a rather large dose of melatonin; many times what our bodies produce.
If 5mg isn't working for you 10mg probably won't either. If you have trouble sleeping you should talk to your doctor.
I didn't ask if it would work I asked if it would be safe! Would it be an overdose or no?
I would not recommend double dosing on melatonin or anything as such unless it is 'okayed' by your doctor first.