Duplicate tasks for same benchmark and para should not be creatable (backend)

[nickvisut]

ACTUAL

If case manager attempts to assign a benchmark to Para "A", then tries to assign the benchmark to Para "A" once more, the database will create a second task with the same benchmark and para

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EXPECTED

If case manager attempts to assign a benchmark to Para "A", then tries to assign the benchmark to Para "A" once more, the app should not allow the case manager to actually create a duplicate task assigned to Para "A". Duplicate rows in the tasks table should not be created no matter what. There should be a uniqueness constraint with the scope of benchmark and para.

[nickvisut]

cc @canjalal

[tam-janice]

Case manager view

Backend Issue: We can create multiple task assigments though the API

UI issue: Separate task - have the UI properly reflect the assign list