Adding message for overwriting a coordinate system

[Ax3man]

When one supplies multiple scales for the same aesthetic, a message is printed that notifies the user. When multiple coordinate systems are supplied, however, the last one is used without notification. This is sometimes confusion for new users, that e.g. expect coord_trans to work with other coord_ functions. Could a message be added?

This gives a nice message:

qplot(1:2, 1:2) + ylim(0, 2) + ylim(0, 3)
# Scale for 'y' is already present. Adding another scale for 'y', which will replace the existing scale.

But this doesn't:

qplot(1:2) + coord_trans(y = 'log10') + coord_polar()

Unfortunately, it's not obvious to me where a replacement could be detected, as there is no coord_list like there is scales_list. I could prepare a PR, but I'd need some pointers.