Will bitcoins be devalued by competing digital currencies? Bitcoin is becoming more recognizable and trusted. But as it's popularity increases, the supply isn't going to change significantly because the supply is fixed to the internal mining algorithm. This is good because it removes the inflationary effect on the currency. Or does it?

Couldn't I just start a new "bitcoin" on a new network, and call it "bitcoins", and therefore double the amount of bitcoins in existence?

That example seems trite, but it gets to a worry I have, that the limited total number of possible bitcoins may be rendered worthless because it is open source and you can just create a secondary bitcoin network to create more bitcoins.

Is this an actual hazard to the value and future growth of the currency? Why, or why not?
You could easily create a bitcoin2, bitcoin3, ... you could create an infinite number of bitcoin currencies having different parametric spaces. These would live in separate mathematical spaces however, so you wouldn't for example be able to create bitcoins in bitcoin2 that affect bitcoins in the original bitcoin: it would be a completely separate system.

But we don't need to be so theoretical about this; there are many alternative digital currencies already in existence:

https://en.bitcoin.it/wiki/Category:Digital_currencies

I would also add other forms of popular credit such as paypal credit, and the new Amazon coin that is coming out to that list.

These can co-exist and there is no problem in this.

I think as a challenger of bitcoin however, it would be difficult, as bitcoin is the most established and most popular digital currency so far. People tend to invest in what is already the most popular, so you get a compounding effect due to this. But this isn't to say that something like something like Amazon coin couldn't eclipse bitcoin for popular use.

Note that these currencies also vary in anonymity and liquidity, which will also affect how popular they become.

But I think in summary I have to say that nobody can predict the future for this kind of thing, so we'll just have to wait and see.