Politics isn’t the only field that has incumbents and challengers.
Real estate 
does, too.
The “incumbent Realtor” is the Realtor who helped the now-selling homeowner buy the home, however many years back; the challengers are all the other Realtors vying for the listing.
Average Realtor “Career”: < 2 Years
I’d guesstimate that something like one-third of the time, the Selling Agent (what Realtors call the Buyer’s agent) on the previous deal pops up again as the Listing Agent when the house comes back on the market.
However, that percentage dwindles over time; after more than a decade, I’d say the odds are less than one-in-ten that the owner goes back to the same Realtor.
The obvious reason: something like 90% of all Realtors last less than 5 years in the business.
P.S.: See, “Realtors vs. Politicians” for other parallels between the two fields.
