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And now eagle-eyed fans have noticed another clue to a past series and it involves one of the prison guards in the latest episode.
Warning - there are spoilers ahead, so don't read on if you haven't caught up with series six episode three yet...
When Farida Jatri is called in for questioning by AC-12, prison guard Alison Merchant goes to collect her.
But Farida doesn't arrive, as the prison guard breaks her wrist while in the cell.
Though shocking in itself, the encounter reminded viewers of something that happened in an earlier season.
Fans were quick to point out that Alison has appeared in the show before - way back in series two, when she attacked Keeley Hawes's character Lindsay Denton to stop her talking to AC-12.
Talk about a coincidence?!
In series two, we saw Alison mix Lindsay's food with faeces, before serving it to her. She later poured boiling water over the detective's hands while warning her not to speak to AC-12.
Shortly afterwards, the prison guard was replaced, never to be seen. Until now.
So what does this mean? Is Alison Merchant on the OCG payroll? It seems likely.
And what does this mean for the investigation? We can't wait to find out more.
Fans on the Reddit thread were shocked by this news.
"She's on the payroll! Bent b***rd," said one.
While another wrote: "Knew I'd seen her face before."
"The OCG is super organised," said a third. "They have key members in positions everywhere they need."
"Can we see these prison guards getting their comeuppance this season please?," a fourth wrote.
Line of Duty continues on Sunday nights at 9pm.
Bent coppers beware!
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