Hi, I have been struggling with my appending results for a while. I appended multiple dtas in the same folder, and I know the observations for each dta, but after appending, the total observations is larger than just adding up the observations of each individual dta. Say, if I have three dtas with 10, 20, 30 observations respectively. After appending, the total observations is more than 60. I use duplicates drop command, and the observations finally matched. I was confused about this situation. Can somebody tell me why sometimes appending yields wrong result in terms of the total observations?
I paste part of my do file in the following:

HTML Code:
cd "mypath"
local dts: dir . files "*.dta"
foreach dt in `dts' {
    append using `dt'
}