I don't see any realistic way to generate an actual alarm from that. The PLC RUN's the code, it doesn't syntactically check it.
The only way to do what you are looking for also doesn't seem worth it, but /shrug...
Get a serious text editor, something like UltraEdit.
Export the PLC program out at every build stage.
Write some code in JavaScript to scan the XML, and parse it for := <sometag>.
Build an array with <sometag> as the index, and an integer value as the data: If <sometag> is not in the array, add it as an index, and set the data property of the array to 1. If <sometag> IS in the array, you have a duplicate assign. Increase the data property, and either flag an error now, or scan the array at the end of the program scan and list the errors then.