feat: Enhance PostHogProvider to prevent double initialization

Added state management to track if the PostHog client has already been initialized, addressing issues that arise when running under React.StrictMode. This change includes warnings for users if the client is loaded multiple times, improving the overall robustness of the PostHogProvider component.

Closes #723

react/src/context/PostHogProvider.tsx

@@ -1,7 +1,7 @@
 /* eslint-disable no-console */
 import posthogJs, { PostHogConfig } from 'posthog-js'
 
-import React, { useMemo } from 'react'
+import React, { useMemo, useState } from 'react'
 import { PostHog, PostHogContext } from './PostHogContext'
 
 type WithOptionalChildren<T> = T & { children?: React.ReactNode | undefined }
@@ -28,6 +28,10 @@ type PostHogProviderProps =
  * but not both simultaneously.
  */
 export function PostHogProvider({ children, client, apiKey, options }: WithOptionalChildren<PostHogProviderProps>) {
+    // Used to detect if the client was already initialized
+    // This is used to prevent double initialization when running under React.StrictMode
+    const [alreadyInitialized, setAlreadyInitialized] = useState(false)
+
     const posthog = useMemo(() => {
         if (client) {
             if (apiKey) {
@@ -50,11 +54,27 @@ export function PostHogProvider({ children, client, apiKey, options }: WithOptio
         }
 
         if (apiKey) {
+            // If the client was already initialized, return the existing instance
+            // This is likely to occur when someone is developing locally under `React.StrictMode`
+            //
+            // There's no built-in way to detect React.StrictMode, so we set a flag to check if the client was already initialized
+            // and return the existing instance if it was.
+            if (alreadyInitialized) {
+                return posthogJs
+            }
+
+            // If it's the first time running this, but it has been loaded elsewhere, warn the user about it.
             if (posthogJs.__loaded) {
                 console.warn('[PostHog.js] `posthog` was already loaded elsewhere. This may cause issues.')
             }
 
+            // Init global client
             posthogJs.init(apiKey, options)
+
+            // Keep track of whether the client was already initialized
+            // This is used to prevent double initialization when running under React.StrictMode
+            setAlreadyInitialized(true)
+
             return posthogJs
         }