is it possible to add the same template multiple times

[Steelfiredragon]

namely the giant template, to increase the creatures size mutiple times??

make a weasel large enough to be used as a mount for a medium sized humanoid??

[LazarX]

No.

[MC Templar]

If you are a GM making an enemy for the player... sure knock yourself out.

If you are a player trying to manipulate the rules... reference LazarX's thoroughly accurate answer above