remove duplicate word

Author: coke

doc/Language/system.pod6

@@ -53,7 +53,7 @@ the default system shell.
 
 The key class for running all external programs is L<Proc::Async>, which runs
 processes asynchronously and allows
-L<concurrent|/language/concurrency#Proc::Async> concurrent interaction with the
+L<concurrent|/language/concurrency#Proc::Async> interaction with the
 running processes. In general, it is a good practice to interact with the system
 through these high-level, abstract interfaces. However, Perl 6 provides with
 other ways of interacting with the system through a low-level interface.