use AppleScript version "2.4" — Yosemite (10.10) or later
use framework "Foundation"
use framework "Quartz"
use scripting additions

script pdfStore
　　property aList : {}
end script

set (aList of pdfStore) to {}

tell application "Skim"
　　set dCount to count every document
　　if dCount = 0 then return
　　
　　tell front document
　　　　set myPath to path
　　　　set pCount to count every page
　　end tell
end tell

set anNSURL to (current application’s |NSURL|’s fileURLWithPath:myPath)
set theDoc to current application’s PDFDocument’s alloc()’s initWithURL:anNSURL

repeat with i from 0 to (pCount – 1)
　　set aPage to (theDoc’s pageAtIndex:i)
　　set tmpStr to (aPage’s |string|())
　　set the end of (aList of pdfStore) to (tmpStr as string)
end repeat

set resList to {}
repeat with i from 1 to (pCount – 1)
　　set aText1 to contents of item i of (aList of pdfStore)
　　set aText2 to contents of item (i + 1) of (aList of pdfStore)
　　
　　if aText1 = aText2 then
　　　　set the end of resList to {i, i + 1}
　　end if
end repeat

if resList is equal to {} then
　　display notification "PDFに重複ページは見られませんでした。"
else
　　set aText to listToText(resList) of me
　　set the clipboard to aText
　　display dialog "重複ページ：" default answer aText with icon 1 buttons {"OK"} default button 1
end if

on listToText(aList)
　　set listText to {"{"}
　　set quotChar to ASCII character 34
　　set firstFlag to true
　　
　　repeat with i in aList
　　　　set j to contents of i
　　　　set aClass to class of i
　　　　if (aClass = integer) or (aClass = number) or (aClass = real) then
　　　　　　set the end of listText to (getFirst(firstFlag) of me & j as text)
　　　　　　set firstFlag to false
　　　　else if (aClass = string) or (aClass = text) or (aClass = Unicode text) then
　　　　　　set the end of listText to ((getFirst(firstFlag) of me & quotChar & j as text) & quotChar)
　　　　　　set firstFlag to false
　　　　else if aClass is list then
　　　　　　set the end of listText to (getFirst(firstFlag) of me & listToText(j)) –ちょっと再帰処理
　　　　　　set firstFlag to false
　　　　end if
　　end repeat
　　
　　set the end of listText to "}"
　　set listText to listText as text
　　
　　return listText
end listToText

on getFirst(aFlag)
　　if aFlag = true then return ""
　　if aFlag = false then return ","
end getFirst