Deleted a harvest source but gives error that resource already exists

[hussnainwithss]

CKAN version
2.8

Describe the bug
I created a harvest resource first which was wrong, hence i deleted it and try to recreate the same one but i get the error that
URL: There already is a Harvest Source for this URL (& config): url=https://data-atgis.opendata.arcgis.com/data.json config=
when there is not
Steps to reproduce

• create a harvest resource
• delete it and remove all its contents
• recreate the same harvest source
• it will give error

Expected behavior
it should have created it as the previous one is deleted.

Additional details

[tomhukins]

This looks like a duplicate report of issue #449.