If I were to use, for example, a poison that gave 100% weakness to poison, and then used another identical poison, would it just replace the first one? Or would it give the target 200% weakness? Or would the first one effect the second one, and give it 300%?

It's making my mind hurt just thinking about it

Hit it with a sword hard. All your problems will be solved-it's like a good massage!

You can only poison your weapon with one poison at a time, if thats what you mean.

My memory is fuzzy, but the weakness to poison has a duration. He's asking whether his first application of the weakness to poison, will be overridden or stacked with his next application.

Weakness to poison is not a "poison" effect; does not stack with itself;
It stacks with weakness to magic probably, however.