Prevent duplicate entries in Log

it fixes the issue I want to be able to prevent duplicate SQL text field rows. That is, if row 1 has the name field already defined as "John Smith", I don't want it to be able to add another "John Smith" (as common as that name might be).

CREATE UNIQUE INDEX idxname ON tablename (fieldname);

it fixes the issue I would check to see if there exists a calendar entry with the same start/end time before you write anything to the calendar. That would make sure that the same entry is never written twice, even if the app is uninstalled/reinstalled.

it should still fix some issue You are probably running into a race condition where two people execute the script simultaneously. When the select is performed, the entry does not exist.
The best way to solve this is to add a UNIQUE constraint on your number field in MySQL. This will make sure the condition is matched at insertion. The query will fail, so you will need to handle the error.

I wish this help you Please note that the term website is misleading when referring to the entries of the hosts file. The entries of hosts file are used for custom mappings of DNS host names to IP addresses, and any host name that is present in the file does not necessarily hosts a website. Using the term website may lead to the false impression that something like http://www.example.com can be added to hosts file which is not true.
Skipping a host if it is already present in the hosts file:

FIND /C /I "%BLOCKEDSITE%" %WINDIR%\system32\drivers\etc\hosts

findstr /IRC:"^ *127\.0\.0\.1 *example\.com *$" "%WINDIR%\system32\drivers\etc\hosts"

if "!Line!"=="!LINE:%BLOCKEDSITE%=!" echo(!Line!>>"%HOSTSFILE%"

findstr /VIRC:"^ *127\.0\.0\.1 *example\.com *$" "%HOSTSFILE%" > "%HOSTSFILE%.tmp"
del "%HOSTSFILE%"
ren "%HOSTSFILE%.tmp" "hosts"

@echo off
setlocal EnableExtensions DisableDelayedExpansion
title Modifying your HOSTS file
color F0

set "HOSTSFILE=%WINDIR%\system32\drivers\etc\hosts"
set "HOME=127.0.0.1"
set "PROMPT_TEXT=Enter the host name to"
set "ACTION_TEXT[1]=add"
set "ACTION_TEXT[2]=remove"
set "FindEmptyLine=^ *$"
set "NewLineAppended="
cls
setlocal EnableDelayedExpansion
:LOOP
echo,
echo 1. Block a host
echo 2. Remove a blocked host
echo 3. Exit
choice /C "123" /N /M "Choose an item [1, 2, 3]: "
set "Item=%errorlevel%"

goto choice%Item%

:choice0 // User Pressed CTRL-C
:choice3
exit /b

:choice1
call :Common
set "HostEntry=!HOME! !HOST!"
findstr /IRC:"!FindEntry!" "!HOSTSFILE!"> nul && (
    echo The host !HOST! is already blocked, No action taken.
) || (
    if not defined NewLineAppended (
        REM This will append a new line ONLY if the file does not end by LF character
        type "!HOSTSFILE!" | findstr $ > "!HOSTSFILE!.tmp" && (
            del "!HOSTSFILE!"
            ren "!HOSTSFILE!.tmp" "hosts"
            set "NewLineAppended=1"
        )
    )
    echo !HostEntry!>>"!HOSTSFILE!"
    echo The host !HOST! blocked
)
goto LOOP

:choice2
call :Common
findstr /VIR /C:"!FindEntry!" /C:"!FindEmptyLine!" "!HOSTSFILE!">"!HOSTSFILE!.tmp" && (
    del "!HOSTSFILE!"
    ren "!HOSTSFILE!.tmp" "hosts"
    echo The host !HOST! unblocked
)
goto LOOP

:Common
set "HOST="
set /P "HOST=!PROMPT_TEXT! !ACTION_TEXT[%Item%]! (e.g. example.com): "
if not defined HOST (
    (goto)2>nul
    goto LOOP
)
set "FindEntry=^^ *!HOME! *!HOST! *$"
set "FindEntry=!FindEntry:.=\.!"
exit /b