Reserved work item replaced

Hi Experts,

The workflow log shows as Reserved work item replaced. Could you please explain what is the actual problem.the workflow status shown as completed.

the status as follows

Dialog work item created                 26.04.2016 23:09:41

Execution started                        27.04.2016 11:03:46

Dialog work item executed                27.04.2016 11:04:15

Reserved work item replaced              27.04.2016 11:04:17

Execution started                        27.04.2016 11:19:36

Dialog work item executed                27.04.2016 11:20:09

Reserved work item replaced              27.04.2016 11:28:01

Work item reserved                       27.04.2016 11:28:02

Reserved work item replaced              27.04.2016 11:28:03

Execution started                        27.04.2016 13:51:16

Dialog work item executed                27.04.2016 13:51:50

Reserved work item replaced              27.04.2016 13:51:51

Execution started                        27.04.2016 14:54:53

Dialog work item executed                27.04.2016 14:59:47

Reserved work item replaced              27.04.2016 14:59:52

Thanks in advance..

Regards,

Upendra.