I'm currently working on an small project and want to know, if the font which you use in the logo, should also be used as, for example, headlines?
There is no rule for or against this, you as the designer can set the rules as you wish when building the corporate identity. Generally however, but again this is not a rule, the typeface in a logo is either a custom lettering or just a different font from typefaces introduced for headlines, body text and so on.
I've had a similar dilemma and this question and answers given may be relevant: How to explain to a client the font in their logo is not to be used for anything else
Though there really aren't rules or guidelines for this in any serious way, I can answer that for myself, I tend to reserve the typeface of a logo to itself - especially if it's a logotype (typographic logo) word-mark or similar.
I'll carefully pair the correct typefaces to work well with, to support, and not fight with, the typeface in the logo, but almost never would I simply repeat the logo's typeface as a headline.
Hope that helps.