C# windows form

Inside of my windows form why is my drop down list repeatings values thats already inside of it when i click a different control on the same form? For example in the same drop down list, I have animals like cat, dog, bird, fish. Say I click bird, then I click any random control,then lets say I click my undo control on my form I created, and then when I go back on my drop down list button I noticed that the animals names are duplicated on that same drop down list. For example

cat

dog

bird

fish

cat

dog

bird

fish what can be the issue code wise or anything?

I dont have the code what might cause it to repeat is it a List or Array issue on the drop down list?