It's important to understand the difference between messages and members. A member exists permanently, but a form submission is only a message that lasts for 90 days.
Many people setup forms as customised registration forms. The first time someone submits a form, any fields preceeded with "from_" will be used to create a member profile, tied to the email address aka "from". On subsequent submissions with the same email address, no member record will be created. So the form can be used for member registration, but each submission is treated primarily as a message.
Each submission of the form does not guarantee a new member record will be created, only if the email address is different, and there will be no warning message. If you do want to allow many submissions with the same email address, then you can download the messages as a CSV file and store them in Excel. Remember you need to do that at least every 3 months because old messages will be deleted.
We have purpose built "member registration forms" and these are tied specifically to the members list, and will produce a warning message if the same email address is used to register twice. These purpose built forms can also have additional custom questions added, and this information will be retained permanently. The only downfall of these member forms is that you cannot control the layout of the fields. Members who attempt to register a second time will be offered the option to receive a password, they can then login and update their member profile that way.
Remember that member records can be created by several means, not just forms or member registrations, but also online shopping, forums, and contact form enquries. This is done specifically so that there is only 1 member record to ensure that all a members activities can be tied together.