In Lunar Laser Ranging (LLR), higher repetition rate is an important trend for development, which is also of great significance for acquiring more LLR data. Based on the previous 10 Hz ranging rate system, a 100 Hz lunar ranging system with common-optical-path structure was developed by Yunnan Observatories, CAS. However, overlapping of the transmitting/echo signals exists in such a common-optical-path system, and seriously disturbs echo-photon detection. In the paper, the overlapping phenomenon was analyzed from the perspective of time sequence, and the correlation between the overlapping and the Time of Flight (TOF) was studied. Overlapping in the system was simulated with the orbital prediction of a lunar retro-reflector of a certain day, and the general overlapping ratio was approximately 8% for the observational duration. Sectional simulation based on each transmitting epoch was also performed, its result was consistent with the previous one. The method of avoiding overlapping by slightly adjusting rotating-mirror''s speed was analyzed, and corresponding experiment was carried out, as proper speed adjustment turned out to be a solution when the overlapping happened.