fix: fix the case where the second slice has duplicate elements

[yafengio]

Description (what this PR does / why we need it):

For the case below:

Union[int]([]int{0, 1, 2}, []int{0, 1, 2, 3, 3})

the output now is：

[0, 1, 2, 3, 3]

and should be:

[0, 1, 2, 3]

What this PR fixes (resolves / be part of):

• fix: fix the case where the second list has duplicate elements.
• feat: add memory prealloc.
• style: code format.

Other special notes for reviewer: