more than one grouped free functions doesn't render

If you group 2 friends, it works. But if you try to group another 2 friends, it stops working.
```
struct T {
friend void f1a();
friend void f1b();
friend void f2a();
friend void f2b();
friend void f3a();
friend void f3b();
};

///@{
/** First group
*/
void f1a();

void f1b();
///@}

///@{
/** Second group
*/
void f2a();

void f2b();
///@}

/** Third single function
*/
void f3a();

/** Third single function
*/
void f3b();
```

Result: The 2nd element of each group doesn't have a javadoc. If you remove the 2nd group markings, then the 2nd element of the first group renders. If you can't repro this I can show you in the actual library where it happens, in case it is contextual.