You should only use a condom one time. If you are going to continue to have sex you should discard the first one and use a new one. If you use the same one over again, the additional fluid in the condom can cause it to slip off in the middle of sex and then the fluid can leak out - which totally voids the reason for using a condom in the first place.

Yep. Condoms are made for one time use, whether you just keep going or not. You should take it off as soon as you are done.