How to find the same item in an array with Ruby -

i have array this.

@arr  =  ["ac", "ba", "ca", "dd", "ac", "bd", "ca", "dd"]  

i want swap letter if first 1 capital. in case, ac becomes ca, ba -> ab etc. becomes this.

@arr = ["ca", "ab", "ac", "dd", "ac", "bd", "ca", "dd"]  

then want find same item. in case there two, ca , dd.

@newarr = ["ca", "dd"] 

thanks in advance.

====

this got far.

@firstarr = @arr.map{|item|        if item[0] =~ /[a-z]/         item = item[1]+item[0]       else         itme = item       end     } 

this gives

@firstarr = ["ca", "ab", "ac", "dd", "ac", "bd", "ca", "dd"] 

for reversing criteria:

foo = @arr.map |x|   (x[0].downcase!).nil? ? x : x.reverse end # => ["ca", "ab", "ac", "dd", "ac", "bd", "ca", "dd"] 

downcase! returns nil if receiver in downcase, using this, above code checks if x[0] can downcase-d; if yes, means first character in uppercase , reverse word, else returns same word.

for duplication criteria:

foo.select { |x| foo.count(x) > 1 }.uniq # => ["ca", "ac", "dd"]