I don't understand the behaviour of the following:

test0[x_, y_] := Module[
   {},
   a = 2*x;
   Return[y]
   ];

test1[] := Module[
   {},
   a
   ];

Now I run this little programms:

In[168]:= Remove["a"]
test0[1, test1[]]
test0[2, test1[]]
test0[3, test1[]]

Out[169]= 2

Out[170]= 2

Out[171]= 4

If "a" ist not defined, "a" will be evaluated in bevor calling test1[], but if "a" has already a value then "a" is evaluated after calling test1[]. How can I force that "a" ist always evaluated before?