Select rows with same id but different value in another column

Question

I've read numerous threads and tried for hours, but I still don't know how to handle this request:

I have a table like this:

+------+------+
|ARIDNR|LIEFNR|
+------+------+
|1     |A     |
+------+------+
|2     |A     |
+------+------+
|3     |A     |
+------+------+
|1     |B     |
+------+------+
|2     |B     |
+------+------+

I want to choose the ARIDNR that appears several times with various LIEFNR.

The output should be something like:

+------+------+
|ARIDNR|LIEFNR|
+------+------+
|1     |A     |
+------+------+
|1     |B     |
+------+------+
|2     |A     |
+------+------+
|2     |B     |
+------+------+

Can someone please help me with this?