Using distinct with stuff/for xml path('')

Use select distinct in the subquery:

select rc.Routage,
       count(distinct rc.Event),
       stuff((select distinct ', ' + cast(rcA.Event as varchar(max))
              from Receiving rcA
              where rcA.supplier = 'user' and
                    rcA.DATETIME > '20170322' and
                    rc.Routage = rcA.Routage
              for xml path('')
             ), 1, 2, '')
from Receiving rc
where rc.supplier = 'user' and rc.DATETIME > '20170322'
group by rc.Routage;

Notes:

• In SQL Server, never use varchar() (or related types) without a length. The default varies by context and you are (potentially) introducing a bug that is really hard to find.
• You want the stuff() to remove two characters, not 1, because you have a comma followed by a space.
• This formulation assumes that Event does not have XML special characters. It is easy to tweak if that is an issue.

Also, this type of query is usually faster if you eliminate the duplicates in a subquery:

select rc.Routage, rc.numEvents,
       stuff((select distinct ', ' + cast(rcA.Event as varchar(max))
              from Receiving rcA
              where rcA.supplier = 'user' and
                    rcA.DATETIME > '20170322' and
                    rc.Routage = rcA.Routage
              for xml path(''), type
             ).value('.', 'varchar(max)'
                    ), 1, 2, ''
            )
from (select rc.Routage, count(distinct rc.Event) as numEvents
      from Receiving rc
      where rc.supplier = 'user' and rc.DATETIME > '20170322'
      group by rc.Routage
     ) rc;

Do the distinct in a subquery, before the XML processing gets anywhere near it:

select rc.Routage
    , COUNT(distinct rc.Event)
    , STUFF((select ', ' + cast(rcA.Event as varchar)

            from (select distinct Event from Receiving a
                  where supplier = 'user'
                   and DATETIME > '20170322'
                   and rc.Routage=a.Routage
            ) rcA

            for xml path(''))
        , 1, 1, '')
from Receiving rc
where rc.supplier = 'user'
    and rc.DATETIME > '20170322'
group by rc.Routage
order by COUNT(distinct rc.Event)desc