For example in the makefile for tlmisc, the install target looks like this:
> install: all
> mkdir -p ...
By having 'all' in the install target, 'all' will be called twice - once for the actual 'all' call, and once for the 'install' call. This can be seen by looking at e.g. the build log from Jenkins and searching for 'make -C tlstunnel'. This also happens for targets tl-setup, profiles and xstartup in the same Makefile, all of which are .PHONY targets.
Greping after '^install[^:]*:' in our tree shows that this type of call is done at least for tlmisc, tlmisc-libs and tl-setup.
For tlmisc, git blame says the line was added 20 years ago, so it has been like this for a long time. It doesn't really cause any issues, but it's a bit unnecessary to build PHONY targets twice.