I have tried to run the Updatemem command twice, but I accidentally left the same output bitstream name both times.
I expect Updatemem to error out with the message that the bitstream exists and to use the -force flag if I really wanted to do this.
However, Updatemem writes the bitstream without the -force flag being required.
The -force flag should be required in order to overwrite an existing bitstream file.
This issue will be fixed in Vivado 2016.1.