fix(payroll): allow duplicate additional salaries

[sagarvora]

As discussed in #24824, it can be useful to have multiple additional salaries created separately:

Take Deadline Incentive for example. It may be desirable to show the same component with possibly the same amount multiple times in the Salary Slip. One for each deadline achieved.

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In any case, this logic shouldn't be in the before_insert method as one can easily circumvent it by saving with a different amount initially and then re-saving with the desired amount.

[Anurag810]

approved