Three Ways to Remove Duplicate Lines from a Large CSV While Keeping Original Order

A developer needed to merge two billing-system exports of customer emails into one CRM import file without repeated entries. Because the CRM rejects files containing any duplicate and gives priority to earlier rows, the deduplication had to preserve original order, ruling out sort -u. The article recommends a Python script that tracks seen lines in a set, and a single-line GNU awk command that accomplishes the same first-occurrence-wins behavior. It notes memory limits for very large files, and mentions a browser-based tool as an option for small pasted lists, processed locally on the user's machine.