Have user confirm input, based on If Else logic

Hi All,
When a user attempts to add something to my page, I need to have to confirm what they are doing if my condition is being met based upon their selected input.
Basically what happens here is that I query my database, return the most common integer from a column, and if they are trying to insert something that doesn't match that column, I want to make them double check that it was intentional (lots of users are forgetting to set the right dropdown value, insertting incorrect data), but the thing is, it's also valid input depending on the situation.
I have a variable, this is the value returned from the database that I have deemed the common value for that column. 
CommonReturnedVar If they select the dropdown value that equals CommonReturnedVar's value, OK, allow them to continue no problems, insert/save data.
however, if the two differ, I want to have the user confirm what they are d ...