### Solve behaviour on same equation twice

Two questions:
**The first one:**
Solving an equation:

```
solve(sin(x)==0,x)
```

gives the solution `[a=pi]`

but

```
solve([sin(x)==0,sin(x)==0],x)
```

raises to the somehow better solution `[[a == pi*z425]]`

Where is the difference between the two equations?

**A second one:** A previous version of sage 5.something could solve

```
solve([sin(x)==0,-sin(x)==0],x)
```

However Sage 6.4.1 Returns an empty list.