This issue generally occurs when a user has upgraded from Lasernet v6.5. The solution, and recommended best practice, is to use different names for the input and output Object Names.
To check this, follow these steps:
1. Open the properties for the rearrange which the sum() function is run against.
2. Check the Object Name on both the Input and Output tabs. Are they both the same value? If so, this is why the total has doubled.