Gazump
make a higher offer for a house than (someone whose offer has already been accepted by the seller) and thus succeed in acquiring the property.
"the trio are fuming after they were gazumped by a property speculator"
The formal usage is as above (and usually illegal) but it also has a metaphorical usage of taking an opportunity that rightfully belongs to someone else.
Cutting someone’s grass
Getting/flirting with someone who your friend has already gotten with or 'shotgunned'.
Also, cut another’s lunch.
Specifically related to dating/sex.