InnoSetup : Problem with file check function - inno-setup

Question

I try to make a setup wizard with InnoSetup software to deploy easily my program but I encounter currently a problem with my script.
I am used to make basic setup wizard but now I have to implement a new functionality whose :
I want to check if a file exist on the computer to not overwrite it.
I made like this (in the [files] section):
[Files]
Source: "{#MyAppExeFile}"; DestDir: "{app}"; Flags: ignoreversion
Source: "{#MyAppSrcFolder}*"; DestDir: "{app}"; Flags: ignoreversion recursesubdirs createallsubdirs
Source: "{#MyAppSrcFolder}FileFolder\testFile.py"; DestDir: "D:\Users\{username}\myFolderTest\"; Flags: ignoreversion recursesubdirs createallsubdirs;Check: needInstallFile(ExpandConstant('{username}'))
And i added this following [code] section:
[code]
function needInstallFile(user : string) : Boolean;
begin
if IsTaskSelected('initFile') then begin
if FileExists('D:\Users\' + user + '\myFolderTest\testFile.py') then begin
MsgBox('The file already exist', mbInformation, MB_OK);
Result:=False;
exit;
end else begin
Result:=True;
exit;
end;
end;
end;
When i compile my code, it's work but if the file already exist, i have three popup (and not just one like i would want to have). I DON'T understand despite my researches on Internet.
If you have a lead to help me, I'm interested because i'm stuck.
Thank you in advance for your help.
PS : Sorry for my English, i am unfortunatly a french guy.
Thomas