It looks like the store replied with revision 3 and then 2 for two refresh requests, one right after the other. If the store is alternating between 3 and 2, that'd explain the behavior observed.
Jun 20 13:17:58 sark /usr/lib/snapd/snapd[1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/find?q=&select=refresh 319.1011ms 200
Jun 20 13:17:58 sark /usr/lib/snapd/snapd[1351]: daemon.go:181: DEBUG: uid=1000;@ POST /v2/snaps/krita 20.822311ms 202
Jun 20 13:17:58 sark /usr/lib/snapd/snapd[1351]: taskrunner.go:238: DEBUG: Running task 168 on Do: Download snap "krita" from channel "stable"
Jun 20 13:17:58 sark /usr/lib/snapd/snapd[1351]: task.go:250: DEBUG: 2016-06-20T13:17:58+02:00 ERROR revision 3 of snap "krita" already installed
Jun 20 13:18:31 sark /usr/lib/snapd/snapd[1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/snaps 4.388231ms 200
Jun 20 13:19:50 sark /usr/lib/snapd/snapd[1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/find?q=&select=refresh 376.562661ms 200
Jun 20 13:19:50 sark /usr/lib/snapd/snapd[1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/snaps 5.899994ms 200
Jun 20 13:20:05 sark /usr/lib/snapd/snapd[1351]: daemon.go:181: DEBUG: uid=0;@ POST /v2/snaps/krita 13.480955ms 202
Jun 20 13:20:05 sark /usr/lib/snapd/snapd[1351]: taskrunner.go:238: DEBUG: Running task 174 on Do: Download snap "krita" from channel "stable"
Jun 20 13:20:05 sark /usr/lib/snapd/snapd[1351]: task.go:250: DEBUG: 2016-06-20T13:20:05+02:00 ERROR revision 2 of snap "krita" already installed
Jun 20 13:20:08 sark /usr/lib/snapd/snapd[1351]: daemon.go:181: DEBUG: uid=1000;@ POST /v2/snaps/krita 13.795327ms 202
It looks like the store replied with revision 3 and then 2 for two refresh requests, one right after the other. If the store is alternating between 3 and 2, that'd explain the behavior observed.
Jun 20 13:17:58 sark /usr/lib/ snapd/snapd[ 1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/find? q=&select= refresh 319.1011ms 200 snapd/snapd[ 1351]: daemon.go:181: DEBUG: uid=1000;@ POST /v2/snaps/krita 20.822311ms 202 snapd/snapd[ 1351]: taskrunner.go:238: DEBUG: Running task 168 on Do: Download snap "krita" from channel "stable" snapd/snapd[ 1351]: task.go:250: DEBUG: 2016-06- 20T13:17: 58+02:00 ERROR revision 3 of snap "krita" already installed snapd/snapd[ 1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/snaps 4.388231ms 200 snapd/snapd[ 1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/find? q=&select= refresh 376.562661ms 200 snapd/snapd[ 1351]: daemon.go:181: DEBUG: uid=1000;@ GET /v2/snaps 5.899994ms 200 snapd/snapd[ 1351]: daemon.go:181: DEBUG: uid=0;@ POST /v2/snaps/krita 13.480955ms 202 snapd/snapd[ 1351]: taskrunner.go:238: DEBUG: Running task 174 on Do: Download snap "krita" from channel "stable" snapd/snapd[ 1351]: task.go:250: DEBUG: 2016-06- 20T13:20: 05+02:00 ERROR revision 2 of snap "krita" already installed snapd/snapd[ 1351]: daemon.go:181: DEBUG: uid=1000;@ POST /v2/snaps/krita 13.795327ms 202
Jun 20 13:17:58 sark /usr/lib/
Jun 20 13:17:58 sark /usr/lib/
Jun 20 13:17:58 sark /usr/lib/
Jun 20 13:18:31 sark /usr/lib/
Jun 20 13:19:50 sark /usr/lib/
Jun 20 13:19:50 sark /usr/lib/
Jun 20 13:20:05 sark /usr/lib/
Jun 20 13:20:05 sark /usr/lib/
Jun 20 13:20:05 sark /usr/lib/
Jun 20 13:20:08 sark /usr/lib/