Question

1. There used to be a traffic cop on the corner.

2. There used to be a traffic officer on the corner.

3. There used to be a traffic police cop on the corner.

4. There used to be a traffic police officer on the corner.
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Can we use #3 and #4?

Answer 1

1 and 2 are fine.

In 3 and 4, it's redundant to have "police" in there.