strange listing

[pagani laurent]

Why do I get multiple occurences of the same packages when I run

sudo port list installed 

[…]
xz                             @5.2.3          archivers/xz
xz                             @5.2.3          archivers/xz
yasm                           @1.3.0          lang/yasm
yelp-tools                     @3.28.0         gnome/yelp-tools
yelp-tools                     @3.28.0         gnome/yelp-tools
yelp-xsl                       @3.28.0         gnome/yelp-xsl
yelp-xsl                       @3.28.0         gnome/yelp-xsl
zlib                           @1.2.11         archivers/zlib
zmq                            @4.2.5          devel/zmq
zmq                            @4.2.5          devel/zmq
zmq                            @4.2.5          devel/zmq


Are they duplicated installations ? Can I remove the duplicates ? How ?

Thanks,
L.