Steps to reproduce the issue

Create a new tag then save it (keep the tag open, dont save and close)
Check the urls field, you get "{}"
Save the tag again without any modification
Check the urls field, you get "{"0":"{}"}"
Save again
Check the urls fiels, you get "{"0":"{\"0\":\"{}\"}"}"

After multiple save, you get "{"0":"{\"0\":\"{\\"0\\":\\"{\\\\"0\\\\":\\\\"{\\\\\\\\"0\\\\\\\\":\\\\\\\\"{\\\\\\\\\\\\\\\\"0\\\\\\\\\\\\\\\\":\\\\\\\\\\\\\\\\"{}\\\\\\\\\\\\\\\\"}\\\\\\\\"}\\\\"}\\"}\"}"}"

Expected result

Actual result

System information (as much as possible)

Joomla 3.5.1

Additional comments