Is it possible for a section to already have a `key` here?  Will this work sensibly (e.g. letting the random dictionary iteration order to pick which one overrides the other would be a bad thing)?

I think these two examples should have the same effect:
```
# a.cfg
[section]
param = 1
param = 2
```
and
```
# b.cfg
[section]
param += 1
param = 2
```
i.e. latter option overrides the former one.

The following config is subtly different:
```
# c.cfg
[section]
param = 1
param += 2
```
and I would expect it to produce the same as
```
# c1.cfg
[section]
param = 1

# c2.cfg
[buildout]
extends = c1.cfg
[section]
param += 2
```
i.e. `section:param` to be set to '1\n2'.

Do you agree?  Does your PR implement this?