This type chart is for generation 6

Flying Press is the first dual-type move added to Pokemon. However, it is actually just a fighting-type move with special type matchups, similar to Freeze Dry. Though it deals damage as though it were a fighting/flying move, it's only listed as a fighting move, and only fighting-type pokemon recieve the same-type attack bonus(STAB). Because of this, I don't consider it to be a true dual-type attack.

If true dual-type attacks were added to Pokemon, they could be very over-powered. For example, a water/grass attack would deal 16× damage against a rock/ground Pokemon. To remedy this, I have devised two rulesets to limit the effectiveness of dual-type moves.

Fighting/flying is an interesting attack type combination because it can't do more than 2× damage against a single type of pokemon. My first ruleset reflects this by limiting the dual-type attack's effectiveness against a single type to 2×. This makes dual-type attacks function similar to single-type attacks, only capable of dealing 4× damage against dual-type pokemon.

My second ruleset limits the overall effectiveness of dual-type moves to 4×. This means they will also be able to hit single-type pokemon with 4× effectiveness. This takes away the advantage single-type pokemon have over dual-types, making very few pokemon completely safe from 4× effectiveness. This ruleset would change the game more than the previous one, which is why I consider it more interesting.

You could describe the first ruleset as a conservative ruleset and the second one as a liberal ruleset, while no restrictions is just pure anarchy.