There are several reasons different loci can be associated with a gene symbol. Most of them boil down to differences in genome annotation either due to different databases doing different things or differences between databases versions. A symbol could have been associated with a sequence that was then found in multiple copies in the genome or a symbol has been assigned independently to different genes or some data sources use outdated symbols. You also have to decide what a gene is for you. For many biologists, a gene is defined by its symbol so any transcript or protein associated with that symbol, wherever they come from in the genome, are part of the gene, i.e. for all practical purposes, biologists often consider duplicated sequences producing the same products as one gene. Alternatively, a gene can be defined by a set of related transcripts/proteins produced by the same locus. This is the type of gene definition used by Ensembl. In this version duplicated sequences give rise to different genes even if they have the same products. To disambiguate, try to map the symbols to IDs, even from different databases then reconcile the IDs and pay attention to versioning. If you know how current your symbols are, you could also use a contemporary Ensembl version to map them to Ensembl genes, considering that your symbols are/were official gene symbols for the genes at the time. For example, there's currently only one Ensembl gene with official symbol C10orf2, other uses of C10orf2 are as synonyms.