B737 First Officer reported unreliable airspeed indications during cruise climb. Flight crew returned to departure airport and landed safely.

Narrative

After we landed at ZZZZ; the CAPT (Captain) worked closely with our station maintenance to address the problem we encountered. After approximately 3.5 hours of trouble shooting; the maintenance personnel was able to make sure that our pitot tube in fact worked properly. They did not find evidence of blockage or icing from weather in their checks as well. After thorough discussion regarding the state of the airplane and assurances of our fitness for duty; we decided to fly the passengers back to ZZZZ1.The takeoff was uneventful. As we climbed in and out of layers of clouds and weather around FL210; the IAS DISAGREE flag returned. As the PF; the CAPT disconnected the AT (autothrottles); AP (autopilot); and turned off the FD (Flight Director) per the immediate action items of the QRH. It was again very difficult to determine exactly which side was reliable as the differences between the two sides were within 7-8 knots of each other. We brought back our relief pilot soon after that and decided that the weather conditions at ZZZZ1 was still less than optimal compared to the good weather at ZZZZ. We contacted Dispatch and Maintenance Control and made sure there were noting else that we could've done at that point. All 3 pilots agreed that returning to ZZZZ was the safest course of action. We then proceeded to request clearance from ZZZZ2 control back to ZZZZ. As we descended towards ZZZZ; we entered a holding pattern over the waypoint of ZZZZZ to burn off fuel in order to avoid doing an overweight landing. We felt it was not a time sensitive event and that it made sense to burn off the extra fuel. During the hold; we asked approach for a block altitude of 10;000 to 12;000 feet and did some troubleshooting. We noticed that the CAPT side's airspeed climbed as we climbed and the airspeed dropped when we descended. We decided that the FO side was more reliable as the airspeed decreased as we climbed and increased when we descended; as expected. After this; we turned on the FD (Flight Director) on the FO side and I took over the PF duties from the CAPT as he worked with the relief pilot to work through the QRH one more time. This time we used the flaps 30 landing performance and landing distance table for unreliable airspeed. We proceeded inbound and landed on Runway XXL without further incidence.