I wonder how idols can promote multiple brands in the same category?

i guess its up to the brands to decide whether to make that offer of promoting competing products

but if the idol's pull is strong enough, it might not even matter

Depends on the contract. Might be one-off thing. So if celebrities gets offered by another brand of the same category, they are free to accept. The brands can also offer multiple one-off contract.

Quote from Lulu_Key

For key's case, these are not on-off endorsements. He had two of the contracts for more than 2 years and he added the third one this year which is why Im wondering how does it work?

i think brands just love the guy

or maaaaybe, the brands are all under the same umbrella, i havent looked into it

this is me trying to figure out how does wonyoung walking with 19 brands on her?

Quote from Lulu_Key

For key's case, these are not on-off endorsements. He had two of the contracts for more than 2 years and he added the third one this year which is why Im wondering how does it work?

Maybe his endorsements is just for specific products/ product line only? Like a sunscreen for this brand, powder for another, toner for another?