As long as you're interested in the last column, you can do it with
$ sort -k3n test.txt | uniq -f2 -D
1+1 = 2
2x1 = 2
BLABLABLA = 2
Here, the sort option
-k3n causes the file to be sorted starting with the third field, in numeric order; the options to
-f2 Skip the first two fields before checking for uniqueness
-D Print all the repeated lines
Unfortunately, you cannot control the number of fields to be checked for uniqueness. You can use
-w to specify a number of characters to check, but that will only help if the field you are interested in is fixed-width.
Also, beware of trailing whitespace. It will be included in the text to be checked for uniqueness.