profiles: Remove duplicates in manifest transactions.

profiles: Remove duplicates in manifest transactions.

Fixes <https://bugs.gnu.org/23874>.
Reported by Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>.

* guix/profiles.scm (manifest-transaction-effects): Delete duplicates in
install and remove.  Let multiple upgrades and downgrades shadow previous
transactions of the same kind.
* tests/profiles.scm
("manifest-transaction-effects no double install or upgrades")
("manifest-transaction-effects no double downgrade")
("manifest-transaction-effects no double removal"): New tests.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>