Answer set with two abductive supports is duplicated

@iensen :grin:

Below is a CR-Prolog program:

```
predicates
a(). b(). c().

rules

a.

-a :- not b, not c.
b :+. c :+.

b :- c.
c :- b.
```

Sparc+DLV gives:

```
SPARC  V2.49
program translated
{a, b, c}

{a, b, c}
```

The answer set `{a, b, c}` is duplicated, possibly because it has two abductive supports `{b :+.}` and `{c :+.}`.
