From a run with:
 -XX:+UseG1GC -XX:+UseStringDeduplication -XX:InitiatingHeapOccupancyPercent=1 -Xmx256m -XX:+PrintGC

#251: [GC pause (G1 Evacuation Pause) (young) 223M->102M(256M), 0,0625804 secs]
#252: [GC pause (G1 Evacuation Pause) (young) (initial-mark) 223M->104M(256M), 0,0296804 secs]
#253: [GC concurrent-root-region-scan-start]
[GC concurrent-string-deduplication, 14,5K->2760,0B(11,8K), avg 20,2%, 0,0010804 secs]
#253: [GC concurrent-root-region-scan-end, 0,0174551 secs]
#253: [GC concurrent-mark-start]
#253: [GC concurrent-mark-end, 0,1190533 secs]
#253: [GC remark, 0,0587351 secs]
#253: [GC cleanup 156M->156M(256M), 0,0090051 secs]


The "GC concurrent-string-deduplication" line should look like

#253: [GC concurrent-string-deduplication ...