Duplicate dataitems in "Grouping by group", what causes it?

Two possible reason as what causing this issue:

When a sampler was defined twice in Manage Entity, either in type or sampler itself.
When a netprobe running in a machine having with two hostname resolving in one IP address. You may want to refer to gateway.log and look for this kind of entry:

<Sun Feb 15 12:14:34> INFO: HOSTRESOLVER host hostname1.domain.com resolved to 169.94.130.100 <Sun Feb 15 12:14:34> INFO: HOSTRESOLVER host hostname2.domain.com resolved to 169.94.130.100