Bug with checkbox: new option replaces an existing one

Hi,
I’m using the free version of Formidable and I recently found a bug that prevents me from further working with it.

On the website of the company I work for we have mandatory fields that one has to fill in with their contact information and a list of checkbox fields with trips that we organize. The client has to flag from the checkbox list the trips they want to receive information about, we get an email with their data and send them information on the trips they have selected.

However, anytime I wish to edit the list of trips (checkboxes) by adding new trips, the latest option I add is somehow connected to an already existing option in the list. The result is that when I finish and save the changes, the new added option replaces the one that already existed. What I want to achieve is simply the addition of an option to the list.
I noticed that the new and the “existing” options are connected because if I check the new option’s checkbox in the preview screen, also the old one gets checked.
In order to avoid this, I have to add a lot of new options, checking each time if the new one I’m creating is connected to another one by flagging it, and only when by flagging on the new option I get only one flagged (and not two), then I can proceed with the creation of the new option I want to add.

This is of course a cumbersome process and I’d like to know if you have a solution to avoid this or suggestions on how to solve it.

This is the link of page I’m talking about https://www.russiacristiana.org/itinerari/richiesta-informazioni-%e2%80%a2-itinerari/
Here it is also a link to the screenshot of the preview where you can see the checkboxes when I try to add a new option to the list https://www.russiacristiana.org/pdf/screenshot_rc.jpg

Thank you in advance for your help,
Elisabetta

The page I need help with: [log in to see the link]