ICS Merge & Dedupe

About this tool

ICS Merge & Dedupe combines one or more iCalendar files — the .ics format that Google Calendar, Apple Calendar and Outlook export — into a single calendar and removes duplicate events along the way. Paste the contents of one file, or concatenate several files one after another, and you get back one clean BEGIN:VCALENDAR…END:VCALENDAR document with each event kept only once.

Duplicates are matched the way real calendars overlap: by UID when both copies carry one (the same event re-exported from the same source), or by start time and title for the same public event that two different apps each stamped with their own UID. Every surviving event is copied through verbatim — its properties, folded lines, alarms and RFC 5545 escapes are untouched — so the result imports cleanly back into any calendar app. Line endings are normalized to CRLF, identical VTIMEZONE blocks are emitted once by their TZID, and VTODO / VJOURNAL / VFREEBUSY components pass through unchanged.

Everything runs locally in your browser — your calendar never leaves your device.

Worked example

Input — two files that both contain the same event (same UID):

BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
UID:[email protected]
DTSTART:20240309T081530Z
SUMMARY:Team Standup
END:VEVENT
END:VCALENDAR
BEGIN:VCALENDAR
VERSION:2.0
BEGIN:VEVENT
UID:[email protected]
DTSTART:20240309T081530Z
SUMMARY:Team Standup
END:VEVENT
END:VCALENDAR

Output (defaults — smart matching, keep first, sorted) — one calendar, one event:

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//gizza-ai//ics-merge-dedupe//EN
CALSCALE:GREGORIAN
BEGIN:VEVENT
UID:[email protected]
DTSTART:20240309T081530Z
SUMMARY:Team Standup
END:VEVENT
END:VCALENDAR

Options

• Match duplicates by — smart (default) uses the UID when an event has one and otherwise falls back to normalized start + title; uid_start matches UID and start time, so recurrence overrides that share a UID but differ in start are kept; start_title ignores the UID entirely and matches on normalized start + title, collapsing the same event exported by different apps.
• Keep which copy — when a duplicate group is found, first keeps the earliest occurrence in document order, while last_modified keeps the copy with the newest LAST-MODIFIED (falling back to DTSTAMP) so a later edit wins.
• Sort events by start time — on by default; turn it off to keep events in the order they appeared across the input files.
• Calendar name — optionally sets X-WR-CALNAME, the title Google/Apple/Outlook show for the merged calendar.

Limits & edge cases

• Only VEVENT blocks are deduplicated. VTODO, VJOURNAL and VFREEBUSY are passed through unchanged (never merged), and at least one event is required.
• Title matching is whitespace-normalized and case-insensitive, so Independence Day and Independence Day match — but genuinely different wording will not.
• Start times are compared after normalizing, so 20240309T081530Z and its ISO form compare equal. Z (UTC) times are exact; floating and TZID times are read as wall-clock UTC because no timezone/DST database is shipped — the same wall-clock value from two files still matches.
• Recurrence rules (RRULE) are not expanded — a repeating event stays a single master event, and uid_start will not treat its instances as separate.
• Events with no UID, start and title at all carry no identity and are always kept, never merged away.

FAQ