[1] - If there's a post that I've made on a topic and I want to copy-paste the same content in a different thread of mine, then is it allowed?
Yes, it's allowed, as long as it's not a topic identical to your previous one (what would be the use, anyway, to have a duplicate topic?). But you can quote yourself, if that's what you mean. Or copy / paste parts of your topic, if they are relevant for a new topic.
[2] - If there's a post that I've made containing content from an external source but I've mentioned the source link in the first post and I'm sure I'm going to make a second post in the future related to the content of the external source.
Now in the future if I decide to copy-paste the content from an external source but with the same link, then do I have to mention the source link again? (forgetting to mention the source link again will result in a ban?)
It would be best if you mention something like "as the above mentioned site states, it continues with the following"... Practically, if you make new posts to your topic, still quoting an external source, you should mention that what you say in the new post is still not your content, but someone else's.