Why are products added into the listing in loops?

The problem you have faced is related to the incorrect work of the Two-Level Cache (Apc + File) system in your Magento. So the data which was already accessed before might be loaded again. This usually happens because of the slow backend part. File Cache (slow backend) may often miss some cache values.

The issue with Magento cache affects M2E Pro because Magento tries to escape using tags in its code while M2E Pro actively uses the tags.

As a solution, we recommend you to disable Memcached as 'Slow Cache Backend' as it does not support tags and leads to the cache tag conflict, consequently.

To disable the Memcached, please navigate to System > Tools > Cache Management and remove 'slow_backend' and 'slow_backend_options' nodes.

Please remember to flush Magento cache after the options are removed from Magento configurations.

Please let us know if you find this information inaccurate