Feature Request: Exception or Warning on Duplicate Configuration Items

[HairlessVillager]

Currently, py-dotenv uses a strategy of overwriting with the latter value when handling configuration items with the same key, that is, using the last value of the repeated key. However, I just wasted an entire morning because there were three configuration items with the same key in my .env file. 😢

To avoid others encountering similar issues, I propose adding an optional behavior parameter. The default should be warning, meaning the latter value overwrites the former but with a warning. It could also be set to strict, which throws an exception, or to latter-covers, which means the latter value overwrites the former without any warning.

I have researched how other libraries handle this issue, for reference:

Parser	Duplicate Handling	Notes
Node.js dotenv	Latter covers	Most common mode
Python dotenv	Latter covers	Configurable to override system variables
PHP phpdotenv	Default error	Can be set to allow overrides
Ruby dotenv	Latter covers	Issues a warning
Go godotenv	Latter covers	No warning
Docker Compose	Latter covers	Follows file loading order

Append: maybe a Callable[[list[str]], str] like sum is also acceptable, for #374 suggests a customed merge logic.