and WP Login Lockdown but I've now found the answer: they seem to be exactly the same plugin.

https://getwpcaptcha.com/free-vs-pro-features-comparison/
https://wploginlockdown.com/free-vs-pro-features-comparison/

Both products have the same feature table and licencing plans. The only difference seems to be that WP Captcha includes some captcha functionality in the free version, which WPLL doesn't.

From this, I assume that if I have already paid for one plugin, I don't need to buy the other.

Out of curiousity, what is the point of selling the same plugin functionality under two different names? Why not just discontinue one of the products and transfer active licences to the other? Will one be better supported ongoing? Are they sharing things like IP blacklists?

Thanks.