Why copy file failed when I copyed many times

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C#

.NET

when i use File.Copy(path1,path2,true) function to copy file many times it failed。
path2 has empy content,and i use API GetLastError,the return value is zero.
or use try catch,but it didn't throw exception.
and then i open the directory with path2,the file content is empty,why?
how shall i do can success copy file any times?

I review my code about path1,and path2,i'm sure the path is correct and has no mistake.
I just do two steps:
(1).opendialog and set path1 for example c:\a.txt ,copy it to path2 for example c:\temp.txt.
(2).load file second times,opendialgo and set path1 to new value,for example c:\b.txt.
copy it to path2 c:\temp.txt
but second times, temp.txt is empty content,that means copy failed,why,i cann't understand the reason.

What I have tried:

try
{
   File.Copy(filePath, tempPath, true);
   uint iiii = GetLastError();
}
 catch (Exception e)
{
    throw e;
}
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