"Cannot resolve collation conflict for equal to operation" error

I get this error when running the query below.
"Msg 446, Level 16, State 9, Line 1
Cannot resolve collation conflict for equal to operation"

I have checked that all referenced columns and tables have the same collation - and they do:
SQL_Latin1_General_CP1_CI_AS

so why do I get this error, and more importantly, how can I resolve it?

select distinct company = p1.[name]
,            SIC = p1.sic
,            Employees = p1.emps
,            case when p1.duplicate is not null
                  then p2.[key]
                  else p1.[key]
            end
,            t.[registered number]
from Primary_Dupelist p1
left join primary_dupelist p2
      on p2.[duplicate] = p1.[key]
left join e_corporate_tree t
      on t.[registered number] = substring(p1.regno,2,len(p1.regno))
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