In collaborative software development, changes made in parallel by multiple developers may conflict. Previous research has shown that conflicts are common and occur as textual conflicts or semantic conflicts, which manifest as build or test failures. With many parallel changes, it is desirable to identify conflicts early and pinpoint the (minimum) set of changes involved. However, the costs of identifying semantic conflicts can be high because tests need to be executed on many merge scenarios. We propose Semex, a novel approach to detect semantic conflicts using variability-aware execution. We encode all parallel changes into a single program, in which "if" statements guard the alternative code fragments. Then, we run the test cases using variability-aware execution, exploring all possible concrete executions of the combined program with regard to all possible merge scenarios, while exploiting similarities among the executions to speed up the process. Variability-aware execution returns a formula describing all failing merge scenarios. In our preliminary experimental study on seven PHP programs with a total of 50 test cases and 19 semantic conflicts, Semex correctly detected all 19 conflicts.