[SPARK-19940][ML][MINOR] FPGrowthModel.transform should skip duplicated items

## What changes were proposed in this pull request?

This commit moved `distinct` in its intended place to avoid duplicated predictions and adds unit test covering the issue.

## How was this patch tested?

Unit tests.

Author: zero323 <zero323@users.noreply.github.com>

Closes #17283 from zero323/SPARK-19940.