Stockholm City Archive Launches Push to Purge Thousands of Duplicate Images From Its Digital Collections

Stockholm's Stadsarkivet confirmed this week it is moving forward with a systematic duplicate-image replacement program across its online digital archive, a project that has been quietly in preparation since early 2025 and has now entered its active remediation phase. The archive, housed on Kungsklippan in Kungsholmen, holds more than 1.8 million digitised photographs, maps, and documents — a collection that has grown faster than the tools used to maintain it.

The timing matters. Stockholm Stad is midway through a broader digital infrastructure overhaul under its Digitaliseringsprogram 2024–2028, which commits city departments to improving the findability and integrity of public records. Duplicate and low-quality images clog search results, increase storage costs, and — critically — can cause confusion when historical records are used by planners, journalists, or researchers who assume each image in a search result is unique.

What the Review Actually Involves

The duplicate-image replacement work is not a simple deletion exercise. Staff at Stadsarkivet are cross-referencing entries using perceptual hashing software to identify images that are visually identical or near-identical but have been catalogued under separate accession numbers — sometimes years apart, sometimes by different digitisation contractors. Where a duplicate is confirmed, the lower-resolution or more poorly described version is flagged for replacement or consolidation under a single canonical record.

Kungliga Tekniska Högskolan, which has an ongoing research partnership with the city on archival machine-learning tools, is providing technical support for part of the identification pipeline. The university's division working on the project is based at its Campus Valhallavägen site in Östermalm. According to documentation published by Stadsarkivet earlier this year, the first phase of the review covers photographs taken between 1880 and 1960 — a period when the same image was frequently printed, donated, and re-digitised multiple times from different physical copies held at different institutions.

The archive estimated in its January 2026 project brief that between 12 and 15 percent of images in that date range may carry at least one duplicate entry. Across a collection of that size, that represents tens of thousands of individual records requiring human review before any replacement or suppression is made permanent. Staff have been allocated an initial budget of 2.4 million kronor for the first remediation phase, which is scheduled to run through December 2026.

Why Researchers and Planners Are Paying Attention

The practical stakes go beyond housekeeping. Stadsbyggnadskontoret, the city planning office on Fleminggatan in Kungsholmen, routinely draws on Stadsarkivet holdings when assessing proposed developments in historically sensitive areas such as Gamla Stan, Södermalm, and the Djurgården shoreline. Planners who pull archive photographs to document the historical character of a building or streetscape need confidence that what they are looking at is a distinct evidentiary record, not the fourth copy of the same glass-plate negative.

Several heritage organisations, including Stockholms Stadsmuseum on Södermalmsallén, have also flagged the duplicate problem in consultation documents over the past two years. The museum's own photographic holdings overlap with Stadsarkivet's in places, and aligning the two collections is part of a longer-term goal that the current deduplication work is intended to support.

For members of the public who use the archive's free search portal — Stockholmskällan — the most visible change will be cleaner search results and better-quality lead images on historical records. The portal currently serves roughly 40,000 unique users per month, according to figures cited in the archive's 2025 annual report.

Stadsarkivet says the public portal will begin reflecting the cleaned records in rolling updates from September 2026 onward. Researchers who have previously saved links or citations to records that are consolidated may need to update their references, as some accession numbers will be deprecated. The archive has committed to maintaining redirect links for at least three years after any record is merged or suppressed. Anyone with questions about specific records can contact the archive directly through its reading room at Kungsklippan, which is open Tuesday through Friday.