code: function verifyEmitFilePath(emitFileName, emitFilesSeen) {
                  if (emitFileName) {
                      var emitFilePath = ts.toPath(emitFileName, currentDirectory, getCanonicalFileName);
                      // Report error if the output overwrites input file
                      if (filesByName.contains(emitFilePath)) {
                          createEmitBlockingDiagnostics(emitFileName, emitFilePath, ts.Diagnostics.Cannot_write_file_0_because_it_would_overwrite_input_file);
                      }
                      // Report error if multiple files write into same file
                      if (emitFilesSeen.contains(emitFilePath)) {
                          // Already seen the same emit file - report error
                          createEmitBlockingDiagnostics(emitFileName, emitFilePath, ts.Diagnostics.Cannot_write_file_0_because_it_would_be_overwritten_by_multiple_input_files);
                      }
                      else {
                          emitFilesSeen.set(emitFilePath, true);
                      }
                  }
              }
docs: Verify that all the emit files are unique and don't overwrite input files