Try this in a bash script file:

#!/bin/bash



dir1="/REPLACE/THIS/WITH/FULL/PATH/TO/dir1/"

dir2="/REPLACE/THIS/WITH/Full/PATH/TO/dir2/"



echo "Indexing $dir2 ... Please wait ..."



find "$dir2" -type f -follow -exec ls -l {} ; > "dir2_ls_file.txt"



while read f1 f2 f3 f4 f5 f6 f7 f8 f9

                do

                        size="${f5}"

                        name="${f9}"

                        result0=$(find "$dir1" -type f -size "$size"c -follow)

                        result1=$(echo "$result0" | wc -l)

                        result2=$(find "$dir2" -type f -size "$size"c -follow | wc -l)

                                if [ $result2 -gt 1 ]; then

                                        echo "There is more than one file under $dir2 with the same size as $name , so no action is taken!"

                                elif [ $result1 -eq 1 ] && [ "$result0" ]; then

                                        echo "$result0 is the same size as $name , so it will be replaced."

                                #       cp "$name" "$result0"

                                else

                                        echo "There is more than one file or no file under $dir1 with the same size as $name , so no action is taken!"

                                fi

                done < "dir2_ls_file.txt"

If you are satisfied with the result, un-comment cp "$name" "$result0" for the real copying to happen.

Please, first read here for more information and precautions.