Often you will find yourself in a position, where you have two identical races playing against each other. Here is a quick guide on how to use Zerg vs Zerg as an advantage.
When playing against the same race make sure that you know what your opponent is doing. Scout opponent's base frequently to avoid any surprises. You will need to watch out for speedling baneling attack. You must be fast and use every second to build economy and to build units as fast and as much as possible. Build some banelings and speedlings just for case if you will need to counter-attack.
Countering agaist fast tech to mutalisks is quite easy if you know what you are doing and if you are expecting the attack. Use hydralisks to face opponent's mutalisks, they will do their job. Using mutalisks is not a very good idea when facing the Zerg opponent. To create a mutalisk you need 100 gas, which you haven't got. If you do, than you are building too slowly. Hydralisks are much more superior and only take 75 gas to create. But also hydralisk unit can be a disadvantage for its speed. Do not forget for speed upgraded zerglings, which can be also a great counter unit and can be used to harass opponents. Use them and try bring down opponents economy.
When creating the attack force I would recommend to go for roaches, hydralisks. Use them in combination with banelings, which have many tactical advantages explained in other article. Banelings are very important in any battle, whether it is against Protoss, Terran and especially when against Zerg.