You are right. I've peeked what is inside pkgbuild and I didn't noticed, that source urls are different:

http://packages.blackfire.io/binaries/blackfire-agent/$pkgver/blackfire-agent-linux_386
http://packages.blackfire.io/binaries/blackfire-agent/$pkgver/blackfire-cli-linux_386

Thank you for your time.

Request #18299 has been rejected by diabonas [1]:

I fail to see how these package are duplicates, since they download
completely separate sources: "blackfire-agent" is a system service,
while "blackfire-cli" is a command-line tool.

The outdated "blackfire-cli" package should be updated. If the current
maintainer is unavailable, you can file an orphan request to become
the new maintainer.

[1] https://aur.archlinux.org/account/diabonas/