For:

#[test_case(15, 15)]
#[test_case(-15, 15)]

fn test_abs(value: f64, expected: f64) {
   assert_eq!(value.abs(), expected)
}

We get error:

error[E0428]: the name _15_15 is defined multiple times
--> src/test.rs
|
| #[test_case(15, 15)]
| ^^^^^^^^^^^^^^^^^^^^
| |
| previous definition of the value _15_15 here
| _15_15 redefined here
|
= note: _15_15 must be defined only once in the value namespace of this mod