That is a feature that started with AC17 so you might need to read the Migration Document for the full reason.
Basically when importing (or copy paste) from an older version file into AC17 or 18 some materials will be duplicated when they are applied to a different surface. For example you have 2 Surfaces, one red and one blue that have the same 25% fill assigned to them then Building mats will create 2 different Building Materials and append the number. For same Building Mats then you can use the "Delete and Replace" option
Eduardo Rolón AIA NCARB AC27 US/INT -> AC08
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