You can localize a variable with Module:
local[parameter_] := Module[{localVariable},
localVariable = parameter*2;
Return[localVariable]]
No need for Return. In your special case you can achieve the same output with simpler inputs:
local[parameter_] := Module[{localVariable},
localVariable = parameter*2;
localVariable]
local[parameter_] := Module[{localVariable},
localVariable = parameter*2]
local[parameter_] := parameter*2
