[FAQU]

The solution was to index both playermessage and samemessage per client.

Here is the fixed code:

PHP Code:

#include <sourcemod>
#include <sdktools>

#pragma semicolon 1
#pragma newdecls required

char spamcount[MAXPLAYERS + 1] =  { 0, ... };
char playermessage[MAXPLAYERS + 1][128];
char samemessage[MAXPLAYERS + 1][128];

public Plugin myinfo = 
{
    name = "Anti-Spam",
    author = "FAQU"
    //description = "Gags the player for sending the same message 3 times in less than 10 seconds."
};

public void OnPluginStart()
{
    HookEvent("player_say", OnPlayerSay);
}

public void OnClientPutInServer(int client)
{
    ResetSpamCount(client);
}

public void OnClientDisconnect(int client)
{
    ResetSpamCount(client);
}

public Action OnPlayerSay(Handle event, const char[] name, bool dontBroadcast)
{
    int client = GetClientOfUserId(GetEventInt(event, "userid"));
    GetEventString(event, "text", playermessage[client], sizeof(playermessage));
    
    if (spamcount[client] == 0)
    {
        strcopy(samemessage[client], sizeof(samemessage), playermessage[client]);
        spamcount[client]++;
        CreateTimer(10.0, Timer_ResetSpamcount, client);
        return Plugin_Handled;
    }
    
    if (spamcount[client] == 1)
    {
        if (StrEqual(playermessage[client], samemessage[client]))
        {
            spamcount[client]++;
        }
        return Plugin_Handled;
    }
    
    if (spamcount[client] == 2)
    {
        if (StrEqual(playermessage[client], samemessage[client]))
        {
            int userid = GetClientUserId(client);
            ServerCommand("sm_gag #%d", userid);
            ResetSpamCount(client);
        }
        return Plugin_Handled;
    }
    else
    return Plugin_Handled;
}

public Action Timer_ResetSpamcount(Handle timer, int client)
{
    ResetSpamCount(client);
}

void ResetSpamCount(int client)
{
    spamcount[client] = 0;
}