If you enable student to update their choice and set a limit to a choice, if option is full student can't add more options.

Steps to recreate the problem:

• create a choice and set Yes to "Allow choice to be updated", "Allow more than one choice to be selected" and "Limit the number of responses allowed"
• enter two options - set limit for first option to 1 and set limit on second option to 5 for example
• login as student to this course and enter the choice
• select only first option and save selection (first option is now full and no other student can select it, but option tow is empty)
• enter the choice again and select second option also and save selection - you will get an error that the choice is full

Choice should allow student select other options and not to regard already selected option as a new one. Only way for a student to select both options is to delete his choice and select it again. This is not good because someone can fill up his selection in meanwhile.