Singular & plural forms of some nouns are the same.

Singular  Plural  Singular  Plural 
advice advice kennels kennels
aircraft aircraft knowledge knowledge
baggage baggage means means
bison bison moose moose
cod cod music music
crossroads crossroads news news
deer deer offspring offspring
dice dice public public
dozen dozen salmon salmon
fish fish ** scenery scenery
furniture furniture series series
grouse grouse sheep sheep
gymnastics gymnastics species species
headquarters headquarters  staff staff 
information information  swine swine
** More than one fish of the same species . 'Fishes' are the plural of more than one species of fish.