This study applied two methods to assess intra-collection duplication within 339 Nordic oat (Avena sativa L.) accessions preserved by Plant Gene Resources of Canada (PGRC). Putative duplicates, that is accessions carrying a similar accession name, were grouped into 52 duplication groups and included 230 of the 339 Nordic oat accessions. A field assessment based on visual inspection of field plots was conducted during two growing seasons to detect distinct phenotypes within each duplication group. Simultaneously, a descriptor assessment using seven characters with altogether sixteen character states was used in both years for the same purpose. The combined results of both assessments and both years indicated that among the 230 accessions in duplication groups only 118 could be identified as distinct. This would allow for a reduction of 33% of the Nordic oat accessions at PGRC. The field assessment method detected fewer (75%) distinct accessions than the descriptor assessment (84%), when considering all accessions identified as distinct as 100%. Repeatability between years was higher in the field assessment (70%) than in the descriptor assessment (64%). The field assessment requires an experienced germplasm evaluator, but allows for handling large numbers of germplasm accessions and for detecting functional, fitness related, and user-relevant diversity. Combining field assessment with descriptor assessment and more sophisticated methods on selected subgroups may be the most efficient method for determination of internal duplication in genebank collections. Bulking phenotypically similar accessions within duplication groups is preferable to eliminating duplicate accessions when collection rationalization is required, as it reduces the risk of loosing diversity.