OK, so have worked it out finally:
# To externally redirect /testpage.php to /testpage
RewriteCond %{THE_REQUEST} /(testpage)\.php[\s?] [NC]
RewriteRule ^(.*)$ /testpage[L,R=302,NC]
# To internally forward testpage to testpage.php
RewriteRule ^(testpage)/?$ $1.php [L,NC]