Redundant `X | X` rules are allowed and mismanaged.

This is a testcase that demonstrates the problem (in `tests/basic.rs`):
```rust
    multiple {
        A = X:"x" | Y:"x";
    }:
    A("x") => "\
1:1-1:2 => A::X(
    1:1-1:2,
)";
```
The output is the current one and is incorrect: both sides are `"x"` and should match *or* the situation should be statically denied. But only the first one is observed.