I was unable to duplicate the issue. Here are my steps:
1. Create a new CRED form
2. Auto generate form to insert fields
4. Create a new page to host the CRED form. Pages > Add New > Test CRED
5. Insert CRED shortcode and view on frontend
For category I get the following with only (1) bracket (trimmed down):
<input type="checkbox" id="cred_form_192_1-checkbox-3-1479412616" data-wpt-name="category[]" >
I tried again with a custom taxonomy which I called "Double Bracket Test". I added it as Hierarchical. I only got one bracket with that as well (see screenshot).
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I suggest the following:
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Minimal Install
A. Update all Toolset related plugins & WordPress core.
B. Minimal Set Up:
1. All plugins except Toolset related disabled
2. Temporarily change the theme to a default WordPress theme (TwentySixteen)
C. If the issue is gone after Minimal Set Up steps above, activate each plugin one by one to find out which plugin is causing the issue.
Does the issue still persist when you do each step above?
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Regenerate the Form
1. Go to Toolset > Post Forms > (form name).
2. Delete the data in the content box (save a backup file with the code somewhere if there are customization there).
3. Click Auto-Generate Form (see screenshot)
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Please confirm your steps
Take a look at my steps above and make sure it matches your steps. If not, please let me know where they differ.
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Please let me know your results.