Why changeset already in history

Occasionally we are getting   eceptions as noted below and are trying to find out the cause.

] lscm -a n -u y accept -r https://jazz.navcanada.ca/ccm --accept-missing-changesets -o -t _W7FFUGsvEemYXccd87JRLw -c _l0644Mz2EeiC8tqUlEKK8A 2>&1 [2019-04-30T10:05:12.164Z] Root exception: <o:p> </o:p>

[2019-04-30T10:05:12.164Z] com.ibm.team.filesystem.client.FileSystemException: Change set (_l0644Mz2EeiC8tqUlEKK8A) is already in history. <o:p> </o:p>

[2019-04-30T10:05:12.164Z]       at com.ibm.team.filesystem.cli.client.internal.subcommands.AcceptCmd.fetchGapFillingChangeSets(AcceptCmd.java:959) <o:p> </o:p>

[2019-04-30T10:05:12.164Z]       at com.ibm.team.filesystem.cli.client.internal.subcommands.AcceptCmd.generateAcceptParms(AcceptCmd.java:850) <o:p> </o:p>

[2019-04-30T10:05:12.164Z]       at com.ibm.team.filesystem.cli.client.internal.subcommands.AcceptCmd.run(AcceptCmd.java:150) <o:p> </o:p>

… <o:p> </o:p>

[2019-04-30T10:05:12.165Z] Problem running 'accept': <o:p> </o:p>

[2019-04-30T10:05:12.165Z] Internal error occurred <o:p> </o:p>

Can someone elaborate on what is causing this ?