However you could compare moves like tackle with scratch and pound, who is a look alike move. If you use TReecko as an example, you' see that he and his evolutions has claws, and could easily tackle into anything. Howeve he doesn't learn thoose moves because he already have Pound in his arsenal. :) This is a RPG thing, they do it in most games.
For example if Gastly would learn curse for example at level 5 and could only found the toxic TM when he was about to level 40, 35, etc, then he maybe could learn poison gás about to level 25, 18, etc. To mediate beetween the two. Of course,this is only an example. Like for example a pokemon who can learn ember, flametrower, fire blast / Water gun/Water pulse/Hydro pump, etc.