The input should be limited to decimal numbers (one decimal place) in the format of 0 - 0.5 - 1 - 1.5 - 2 - 2.5, etc.
You could use a regular expression to check the format after respondents clicked on "next".
/\d+(\.[05])/
This allows a number (one or more digits) and then optionally ".0" oder ".5".
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