UTS gate codes

Could someone explain what a dumbbell is please?

00 - Valid Ticket
01 - UTS Gate Fault (Invalid Tables)
02 - Wrong Type/Direction of use for this type of ticket
03 - BR only - no LUL validity
04 - UTS Gate Fault (Check Table Error)
05 - Technician signed onto machine
06 - Test Ticket
07 - Code Unreadable (Usually when ticket is upside down)
08 - Recoding Error
09 - Ticket Damaged
10 - Carnet Validated (Carnet Validator Only)
11 - Out of Date
12 - Not Valid Now (e.g.. OPTC before 0930, Freedom pass before 0900)
13 - Additional Fare Payable
14 - Not Valid, and is not excessable
15 - No onward validity
16 - Pass at Invalid station
17 - Excess/platform ticket at wrong station
18 - Station Permit at wrong station
19 - Start Date in future
(20 - Spare)
21 - Ticket already used for entry
22 - Ticket already used for exit
23 - Carnet not validated on entry
(24-30 - Spare)
31 - Invalid From OSI
32 - Invalid Into OSI
33 - Illogical Interchange through gates [1]
(34-40 - Spare)
41 - Ticket used three times in quick succession at same station
(entry-exit-entry or exit-entry-exit or purchase-entry-exit)
42 - Double use in one direction
(43-50 - Spare)
51 - Already used for 1 journey (single) or 2 journeys (return)
52 - Pass Reported Lost or Stolen [2]
53 - Permit Reported Lost or Stolen [2]
54 - Possible Dumbbell
(55-60 - Spare)
61 - Too long spent making interchange
62 - Too long spent on journey (carnets only)
63 - Too long on platform (platform tickets only)
64 - Too long on excess (Fare-Paid tickets only)
65 - Entry time exceeded on platform ticket
(66-72 - Spare)
73 - Ticket given back for possible emergency use (e.g.. substitute bus
service) and is not valid at this station
(74-81 - Spare)
82 + 83 - Illogical Use of Ticket
(84-89 - Spare)
90 - Gates set to reject this ticket type (normally child-lock)
(91-98 - Spare)
99 - UTS Gate Fault (Check Time-out)