That didn't occur in this case. I tried to overwrite an existing memory with different data (in this case a different frequency (all other data was the same) and it didn't prompt either way.
Maybe I wasn't clear. Let me try to restate it:
The only time it currently does the "overwrite check" is when you change a value in the "new location" field that also has its "import" box checked. That means, you've selected your file to import, you see the list of fields, you click in the "new location" field for, say, location 50 and change it to 25. /Then/, if your currently-opened radio already has a location 25, then it will complain.
Now, given that, are you still not seeing the prompt?
How about this concept, if it is not too difficult... Prompt at the time the first record conflict is encountered. Display the old and proposed entries and offer the options "Replace Yes, No, Yes to All, No to All" If Yes or No options are selected the same prompt will occur on the next conflict. The "to All" options would remove the requirement for further prompts for the rest of the Import. This concept would allow you to remove the checkbox all together.
The problem that I see with that is this:
You have a radio with 400 memories programmed, and open another to import from, which also has all 400. It asks you once, and you have to say "don't ask me again" because otherwise your finger will die from clicking "No" 400 times. Then, when you've got them all up, you decide to import a memory into an unused location (328) in your current radio, but actually type 238, but it doesn't warn you because you told it not to... See what I mean? :)