How to Remove Duplicate Lines Without Changing Their Order

Removing repeated lines and preserving a meaningful sequence are separate concerns. If order matters, the safest rule is to deduplicate first according to an explicit survivor rule and leave final sorting off.

Mycelgrid Remove Duplicate Lines keeps the first matching occurrence by default. It can instead keep the last matching occurrence. With Sort final list disabled, the surviving occurrences remain in their retained source order. Sorting is optional and happens only after deduplication.

Keep the first occurrence

Suppose the source is:

apple
banana
orange
banana
pear
apple

Keeping the first occurrence produces:

apple
banana
orange
pear

The first apple and first banana represent their matching groups. Later matches are removed. The remaining list follows the order in which those first survivors appeared.

This is the Tool’s default survivor behavior.

Keep the last occurrence

Sometimes the later copy is the one that should represent a repeated value. For example, the later position may reflect a more recent insertion or a sequence you intentionally assembled.

With Keep last occurrence, the Tool retains the last matching occurrence. Internally it scans from the end and then restores the retained survivors to forward order. The result is not simply the source list reversed.

Consider:

A
B
A
C
B

Keeping first occurrences yields:

A
B
C

Keeping last occurrences retains the A at source position 3, C at position 4, and B at position 5, so the retained forward order is:

A
C
B

The difference is about which occurrences survive and where those survivors appeared in the source.

Matching rules come before survivor rules

First-versus-last only makes sense after the Tool decides which lines match.

With default case-insensitive matching:

Apple
apple
APPLE

forms one matching group. Keep-first preserves the representation from the first retained occurrence; keep-last preserves the representation from the last retained occurrence. If Ignore surrounding spaces is enabled, leading and trailing whitespace has already been trimmed from each prepared line, so the surviving output is normalized at those boundaries.

If you need to inspect how case, boundary whitespace, punctuation, internal spacing, and hidden characters affect equality, read What Counts as a Duplicate Line? Case, Whitespace, Punctuation, and Hidden Characters.

Sorting intentionally replaces source order

Sort final list is a separate post-deduplication step. The Tool first determines the survivors, then sorts those surviving values with locale-aware comparison.

That means “preserve order” and “sort final list” are mutually different outcomes:

• leave sorting off when the retained source sequence carries information;
• enable sorting when alphabetical/locale-aware organization is more useful than source order.

Do not describe a sorted result as preserving original order.

Why source order can matter

Sequence may represent chronology, priority, discovery order, manual arrangement, or workflow order. In those cases, automatic sorting can remove information even though the text values themselves are unchanged.

A duplicate-free list is therefore not automatically a better list if the process also destroys a meaningful sequence.

Preserve the source while you review

The current Tool keeps the pasted source separate from the generated Unique lines result. Use that separation as a review boundary.

A conservative workflow is:

1. Keep the original source available.
2. Define what counts as the same line.
3. Choose whether the first or last matching occurrence should survive.
4. Leave Sort final list off when source sequence matters.
5. Remove duplicates.
6. Compare the survivor order and representation with the source.
7. Use Copy unique lines or **Download TXT** only after the result is correct for the next system.

The downloaded TXT contains the generated unique-line result, not a replacement of the source. The Tool’s source/result separation makes it possible to review before you carry the transformed list forward.

Stop when order is part of the data

If you cannot explain what the source sequence means, do not sort merely to make the list look cleaner. Deduplication answers which repeated entries survive. Sorting answers where the survivors should appear. Keeping those decisions separate is the simplest way to remove repetition without accidentally discarding sequence information.