It's easier to keep track of one item than two of the same item.
When I have one pen, I can remember where I last used it. But when I have two, I eventually lose both. My memories of the second pen interfere with my ability to remember the location of the first pen, and vice versa.
I don't have this problem with different items. I can easily find my pillow, my hammer, and my first-aid kit.
I can use this to my advantage by keeping similar items mentally separate. One pen for work, one for home. A pair of shoes for winter, another for summer.
Also, I can get rid of unnecessary duplicates. One spatula. One alarm clock. What counts as unnecessary depends on the situation. What can I live without if it breaks?
What items do you displace?