onMount is getting fired again on page change (same route)

[shank233]

Describe the bug
I created a sapper svelte application and some weird thing is happening.
Once the page is rendered on the client side from the server, I click on a page "A" which has an component "comp1". This component contains onMount lifecycle method.
Now if I click on page "B" which doesn't have component "comp1" but still it calls the onMount lifecycle method of "comp1".
Now if I click on page "C" (which doesn't have component "comp1") after page "B" then "comp1" onMount method is not called.
If I directly click from page "A" to page "C" then "comp1" onMount method is getting called.
So any immediate click on another page from page "A" calls the "comp1" onMount method.

I have create routes/[...slug].svelte file so any path on the browser will match to this file except / which will map to routes/index.svelte.
Both routes/[...slug].svelte and routes/index.svelte has same code.
Also components are getting rendered dynamically using svelte:component.
_layout.svelte is not getting used and it has been renamed to __layout.svelte.

To Reproduce
"svelte": "^3.17.3",
"sapper": "^0.28.0",

[niklasgrewe]

unfortunately the error occurs with me also that functions are executed twice:

onMount(() => {
    console.log("Hello");
})

Output:

Hello
[SAPPER] dev client connected
Hello

My Environment

• Sapper: v0.28.4
• Svelte: 3.24.1

as i noticed, the error occurs not only in onMount(), but also in async functions, which are executed via on:click={function}.

---

Data stored twice in DB

the biggest problem i have now is that data is stored twice in my database, although i check first if the same data is already stored. This is a real problem for me and honestly I don't know how to fix it. It looks to me as if the function is executed twice at the same time, otherwise my check would work.