Log when truncindex.Get returns >1 container

When the user is not using the full has to retrieve a container it's
possible that we find conflicts with the ids of other containers.

At the moment it's just failing saying that it can not find a container,
but it doesn't say why. Adding a small log saying that duplicates where
found is going to help the user.

Closes #8098
Because it's possible that you get a duplication ID error, but at the same time, if you continue to the `GetByName` you will get a container, so the previous error is not a problem anymore.

If I have this code:

```
if id, err := daemon.idIndex.Get(name); err == nil {
    return ...
} else if err == truncindex.ErrDuplicateID {
    log.Errorf(...)
}

if c, _ := daemon.GetByName(name); c != nil {
    return c
}
return nil
```

And I have this containers:
- by ID: aabc3f & aad9ae
- by Name: aaawesome (yeah, stupid name :D)

When I call this function with `aa` I will get the log error but at the same time I will get the container thank to the `GetByName`.