|Oh Gods, not this again|
What do you do?
Maybe you can see a potential overtaking space up ahead, so you speed up so you're close enough to be able to shoot through when the space appears, without getting so close you're right up behind them like some sort of creeping creeper who creeps.
Maybe you put on your Busy Person With Places To Be face, cough an 'excuse me', elbow through and get on your way. Protip: this works well if you're in a suit or the sort of thing people wear while carrying briefcases. It's a much harder move to pull off in scruffy jeans and a tour t-shirt for a band that split up in 2003.
Or maybe you pull a leaf from the book of what to do when someone else is standing in front of the supermarket shelf you want to look at. That's the point when you feign deep interest in something on another shelf and wait for them to move so you can get to what you're after. So, you stop for a bit to admire the display in an empty shop window or count some marigolds on a roundabout, to put some space between you and them. Then you set off again, catch up again, stall again, set off again.
And then run into them, because while you've been scheming how to get past them you didn't notice they'd stopped to have a chat in the middle of the path.