Banns read multiple times at St Brides, Fleet Street early 19th century

It wasn't unusual for Banns to be read but the marriage not to go ahead -- the couple might have got cold feet, or the groom might have been called away on military service, or parents might object to the marriage of a minor. Do you know when the spouses were born?

Additionally, at the time in question, there was no limit on the period that elapsed between banns and marriage. See https://genealogy.stackexchange.com/a/15755/6485. So the banns being read in 1820 would have legitimised the marriage in 1821.

However, I suppose it's possible (but have no source to back this up) that the vicar was worried about it in hindsight and had the banns reread.

It would be worth working out the sequence of other events for the couple concerned. Were they minors? Might the groom have been called away? Were the baptisms of their children inconveniently close to the marriage date? What else might have been going on? You need more context to sort this out.