This functionality is supported only if you are using the Classic Agent.
The attributes tag does not use the [n] notation to distinguish between windows with the same value of the tag. If the attributes tag is comprised only of an attribute that is not unique, then the same tag is recorded for multiple objects. The duplication is not detected until Silk Test Classic tries to match the tag at runtime, at which time an E_WINDOW_NOT_UNIQUE exception will be raised.
If you anticipate that multiple windows may have the same value for the attributes tag, then use multitags instead (the attributes tag will have precedence).