Update interval is not always centered around 0.5x seconds

Hi everyone,

As ADS-B specification says, the position is updated twice every second. However, when I calculate the update interval by having the difference between consecutive mintime, it seems that some data received almost immediately after the previous one. I tried to collect data for one aircraft  observed by all receiver and one receiver observing all aircraft, both data has this kind of pattern [see figure attached]. Does anybody know what might be the reason for this?

After looking into the data in detail, I found that some rawmsg in the position_data4 table are duplicated, thus making the mintime difference very tiny. This happens for both one aircraft and one receiver observations.vWhen I drop the duplicates in the rawmsg, the weird bump disappear [picture attached]. Could this be a bug?