srml/staking：new_session was called twice almost at the same twice

[leahnoah]

Hi, I run the node based on Substrate, and found the function new_session in srml/staking was sometimes called twice in a row almost at the same time. Would that result in rewarding twice in a row almost at the same time, even the era index not changed？ Or the reward would only happen according to the second time?

Below is the snapshot of the log:
2019-08-20 12:26:36 Pre-sealed block for proposal at 508. Hash now 0x15ff508a9777566da31c07e72de4fe49d87abba69422a61a861317fef49adcf4, previously 0xb12e4a77cf0dc88d0d308a7c5472944578c211b5ccfd90bbe1fe79775e7aeaed.
///// start session index 52
2019-08-20 12:26:36 ////// on session ending
2019-08-20 12:26:36 /////era length 2, sessions per era 2
2019-08-20 12:26:36 force era NotForcing
2019-08-20 12:26:36 /////start session index52
/////the reward points as below ------NO.1
2019-08-20 12:26:36 /////total_points 404
2019-08-20 12:26:36 /////single points 102
2019-08-20 12:26:36 /////single points 100
2019-08-20 12:26:36 /////single points 102
2019-08-20 12:26:36 /////single points 100
2019-08-20 12:26:36 ///// era index 26
2019-08-20 12:26:36 Imported #508 (0x15ff…dcf4)
///// start session index 52 again!!!!
2019-08-20 12:26:36 ////// on session ending
2019-08-20 12:26:36 /////era length 2, sessions per era 2
2019-08-20 12:26:36 force era NotForcing
2019-08-20 12:26:36 /////start session index52
/////this time, the reward points changed ------NO.2
2019-08-20 12:26:36 /////total_points 404
2019-08-20 12:26:36 /////single points 102
2019-08-20 12:26:36 /////single points 100
2019-08-20 12:26:36 /////single points 82
2019-08-20 12:26:36 /////single points 120
2019-08-20 12:26:36 ///// era index 26
/////that means reorg
2019-08-20 12:26:36 Reorg from #508,0x15ff…dcf4 to #508,0xbd4a…5b2b, common ancestor #507,0x4684…362e

My question is:
Is it expected behavior?
Would there be twice reward actions? or only reward NO.1 happened? Or only reward NO.2 happened?

thanks