Validate uniqueness before doing the transform

[chinyeungli]

There are cases that a renamed field is already in the input that is causing issue. For instance,

field_renamings:
        license_expression : 'Concluded License Expression'

If the input already the license_expression field (but it's an empty field), the Concluded License Expression was renamed to license_expression and then this license_expression field got replaced with the original empty license_expression field and therefore the output will contains the orignal empty's license_expression field instead of the value from the renamed Concluded License Expression.

The tool should alret user for the issue and ask user to fix it before doing the transform process

[v-a14]

I want to work on this issue, please assign me this.

[chinyeungli]

Closed #600