In `DATA` below, I was wondering how to find the unique `study_id` in which variable `var1` and `var2` don't form a unique combination?

Obviously, `study_id` that consist of a single row (e.g.,`Bagheri`) always form unique and are to be ignored.

But how could we find unique `study_id` with multiple rows that don't form a unique combination?

**My expected answer is:**

```
> [1]"Alavinia" (because it has multiple rows but keeps repeating the combination `var1 == ext` & `var2 == write`)

> [2] "Busch" (because it has multiple rows but repeats the combination `var1 == ext` & `var2 == speak`)

```

```
m="
study_id  var1  var2
Alavinia  ext   write
Alavinia  ext   write
Alavinia  ext   write
Bagheri   ext   read
Bartz     ext   speak
Bartz     ext   listen
Bartz     ext   write
Bartz     ext   read
Busch     ext   gra
Busch     ext   read
Busch     ext   others
Busch     ext   speak
Busch     ext   speak
"

DATA <- read.table(text = m, h=T)

```