"Block build if parent plans have unbuilt changes" blocking strategy will produce two builds in child plan

Summary

Using "Block build if parent plans have unbuilt changes" blocking strategy will produce two child builds if parent has unbuilt changes and Concurrent builds are enabled

Steps to Reproduce

1. Set Plan A to be the parent of Plan B
2. Remove Plan A trigger
3. Set Plan B trigger to be "Repository polling" (10 seconds)
4. Set "Block build if parent plans have unbuilt changes" blocking strategy in Plan B
5. Set Concurrent builds to be more than 2
6. Push a commit to Plan A
7. Push a commit to Plan B

Expected Results

Plan A will be triggered. After Plan A finish building, Plan B will be triggered once

Actual Results

Plan A will be triggered. After Plan A finish building, Plan B is triggered twice (one from Repository polling and the other from Parent Plan A)