Issue while modifying a custom module created using module b

Hi All,
I have "Small Business Edition" installed of sitefinity with version 5.1. I am facing an issue while updating an existing custom module which I have created. When I am trying to modify existing module to add one more field of media type, it removes associated documents (in this case PDF) for all the records from the content. I am adding a field for Thumbnail image of type Media (images, videos, files) in that module, there is already a field with name 'Document' of Media type. So it might be the case it getting confused which field to use but as the fields are with different name so ideally it should work.
Could you please suggest what I am missing?

Thanks in advance.
Sanjay

Hello Sanjay,

T'his error will be raised if you're trying to create a field with the same name as an already existing field. You mentioned that this is not the case, but please make sure, because we haven't received complains about such problem before. Can you please also attach a video, describing the actions that you take in order to include the custom field?