Beta version of alignDuplicate ‘top’,‘bottom’,‘left’ and ‘right’ in Pixelmator Pro
The approach is to get a mirror of the shape.
I’m welcome for a better version, so do not be shy… 
Make a shape layer and select the layer… run the script…
ex. alignDuplicate:“bottom”
use AppleScript version "2.4" -- Yosemite (10.10) or later
use framework "Foundation"
use framework "JavaScriptCore"
use scripting additions
(**
* [alignDuplicate:(string)]
*)
on alignDuplicate:_align
tell application "Pixelmator Pro"
tell front document to tell current layer
set theName to its name
set {theWidth, theHeight} to {width, height}
set {xPos, yPos} to position
end tell
-- Align to left edge of flip horizontally with duplicate.
if (_align = "left") then
set alignLeftEdge to {xPos - (my doJsMath("abs", theWidth)), yPos}
tell front document to duplicate layer theName
tell front document to tell current layer
flip horizontally
set position to alignLeftEdge
end tell
end if
-- Align to right edge of flip horizontally with duplicate.
if (_align = "right") then
set alignRightEdge to {xPos + (my doJsMath("abs", theWidth)), yPos}
tell front document to duplicate layer theName
tell front document to tell current layer
flip horizontally
set position to alignRightEdge
end tell
end if
-- Align to top edge of flip horizontally with duplicate.
if (_align = "top") then
set alignRightEdge to {xPos, yPos - (my doJsMath("abs", theHeight))}
tell front document to duplicate layer theName
tell front document to tell current layer
flip horizontally
set position to alignRightEdge
end tell
end if
-- Align to bottom edge of flip horizontally with duplicate.
if (_align = "bottom") then
set alignRightEdge to {xPos, yPos + (my doJsMath("abs", theHeight))}
tell front document to duplicate layer theName
tell front document to tell current layer
flip horizontally
set position to alignRightEdge
end tell
end if
end tell
end alignDuplicate:
on doJsMath(theMethod, argsList)
if class of argsList is not list then set argsList to {argsList}
set theContext to current application's JSContext's new()
set theMathObject to theContext's objectForKeyedSubscript:"Math"
return (theMathObject's invokeMethod:theMethod withArguments:argsList)'s toDouble()
end doJsMath