claire ~ % sudo pacman -S ecm gmp-ecm
[sudo] password for claire: 
resolving dependencies...
looking for conflicting packages...

Package (2)        New Version  Net Change  Download Size

community/ecm      1.03-4         0.02 MiB       0.01 MiB
community/gmp-ecm  6.4.4-1        0.48 MiB               

Total Download Size:   0.01 MiB
Total Installed Size:  0.49 MiB

:: Proceed with installation? [Y/n] y
:: Retrieving packages ...
 ecm-1.03-4-x86_64          8.0 KiB  0.00B/s 00:00 [----------------------] 100%
(2/2) checking keys in keyring                     [----------------------] 100%
(2/2) checking package integrity                   [----------------------] 100%
(2/2) loading package files                        [----------------------] 100%
(2/2) checking for file conflicts                  [----------------------] 100%
error: failed to commit transaction (conflicting files)
/usr/bin/ecm exists in both 'ecm' and 'gmp-ecm'
Errors occurred, no packages were upgraded.