by getting the number after they repost
Question: How do they get the number? Answer: "by getting the number" ...
Please try again. And leave off "getting the number" in your response. Because in this case after we remove "getting the number" in your response, we're left with "after they repost". Which still conveys NO information.
I am very tempted at this point to believe that you need to create a method to dedupe before any number is in any way presented to an agent. But since I don't know how it goes from "post" to "agent", it's a general rule that needs to be enforced rather than a "how". Tell me how it gets from POST to Agent and I'll tell you where deduping should happen. Any system can do it if there's any form of automation.
seeing on agents screens the number
Stop showing the number on the agent screen.
They are already talking to the person, they don't need to be staring at the number while doing so. If a situation arises that requires the number, like pushing a web form button, the web form can be programmed to supply the number automatically. And any page that "pops up" does not need to actually SHOW the complete phone number.
Leaving off the last four will foil the nefarious plans of any of your evil roaming call center demons.
Of course, firing one after catching him and docking his pay for fraudulently stealing a commission may awaken the latent honesty of those still in the room. Just sayin'