Giving proper credit for design ideas

I think you should always give proper credit. For example, if you saw a design pattern that worked well on another site, it actually lends credence to give credit to that company for inspiring you - and it may strengthen stakeholders' buy-in, upon seeing something work well for another company, especially if they respect that company. If the other designer is on your team, definitely give credit whenever possible - it will promote good team relations, show that you are someone that can be trusted and a good team player, and getting other people's input into your design usually makes it stronger, as you can't always see what issues there are with your own work. It's a matter of ethics. Everyone builds upon the experience, failures, and successes of others, and those others, if known, deserve our respect.