Make sure config providers are unique

[alexmerlin]

Q	A
Documentation	no
Bugfix	no
BC Break	no
New Feature	yes
RFC	no
QA	no

Description

When initializing the ConfigAggregator, this will iterate through the provided items and look for duplicates. If there are duplicates, a InvalidConfigProviderException exception is thrown with the message:

Some\Provider is registered more than once. Config providers should be unique. In case a specific order is required, please double check before deleting the duplicate(s).