Many tablets are sold at different strengths. For example, Sqad's favourite, Ibuprofen, can be bought in two strengths, 200 mg or 400 mg. If you need 2 x 200 mg to relieve your symptoms then it makes sense to buy the 400 mg packet. Apart from probably working out cheaper it is one less tablet to have to take.
A lot depends on the time lapse between ingestion - even though you've ingested 100gm and even though the 'idiot margin' is fairly wide on over the counter medicines, it doesn't mean that double a recommended dose is the smart thing to do. If it isn't an over the counter medicine then the recommended dosage is there for a reason and should be adhered to. Also, some medicines are not to be taken other than via the instructions and can be affected by lots of things.