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A key defined twice (within one file, or redefined by a later cascade file) is almost always an editing mistake, and the merge is dict-based: the later value silently wins. Nothing crashed, nothing warned, and the failure mode is a silently flipped default discovered in production. The merge semantics are unchanged (last-wins, python-dotenv's own dict behavior); each duplicate now logs a WARNING naming the file, the key, and both values at load time. Within-file detection tokenizes with python-dotenv's own parse_stream (quote- and multiline-aware; a KEY= line inside a quoted multi-line value cannot false-positive), imported lazily inside the detector under a guarded import: dotenv.parser is not public API, so a future python-dotenv may break it - the worst case is that the warning degrades to silence, never an import-time break (the hermetic-loading pin documents the sanctioned exception). Fixes #80
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Closes #80.
What
A key defined more than once in the .env cascade now logs a WARNING at load time, naming the file, the key, and both values:
Duplicate key 'A' in /path/.env: the later value wins. Remove the stale occurrence.Key 'SHARED' redefined in /path/.env.dev: 'base' (earlier file) is overridden by 'dev-specific' (this file).The merge semantics are unchanged (last-wins, python-dotenv's own dict behavior) — this only makes the flip visible instead of silent.
How
Within-file detection tokenizes with python-dotenv's own
parse_stream(one binding per occurrence, quote- and multiline-aware — aKEY=line inside a quoted multi-line value cannot false-positive), imported lazily inside the detector under a guarded import:dotenv.parseris not public API, so a future python-dotenv may break it — the worst case is that the warning degrades to silence, never an import-time break.TestNoDotenvInternalsdocuments the sanctioned exception and still fails on any module-leveldotenv.*import.Tests
tests/test_duplicate_keys.py(5): within-file warn + last-wins unchanged; clean files never warn; quoted multi-line values do not false-positive (and the real duplicate is the only one named); cross-cascade redefinition names both values; three occurrences warn once per key.Full suite: 1,433 passed; ruff clean.