Why does my .sdc file get read twice when I call it using the read_sdc command from another .sdc file?

Due to a problem in the Quartus® II software versions 13.0 and 13.0 SP1, Synopsys Design Constraint (.sdc) files called using the read_sdc command may be read twice during compilation. The problem affects designs using incremental compilation and is specific to cases where the .sdc file is sourced using the read_sdc command from another .sdc file rather than being added to the project.

To avoid this problem, add your .sdc files to your Quartus II project rather than sourcing them from other .sdc files using the read_sdc command.

This problem is scheduled to be fixed in a future release of the Quartus II software.