#2 possible to add same agent and phrase constraints twice

[Bethany Nowviskie]

Unlike other constraints, which are disabled in the right-hand column
after they've been added, it's still possible to add the same agent multiple
times.

It'd also be nice if the system checked if you had already added a
particular phrase constraint and warned or prevented you from adding it
twice -- but this is of lower priority than the agent issue.

[Erik Hatcher]

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The agent part of this has been completed.

[Erik Hatcher]

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Since the year field is now a text field, and soon to be a agent-like suggest drop
down, the same exclusion of existing facet values needs to be taken into
consideration.