Reinventing the wheel
Reinventing the wheel is a phrase that means a generally accepted technique or solution is ignored in favour of a locally invented solution. To "reinvent the wheel" is to duplicate a basic method that has long since been accepted and even taken for granted.
The inspiration for this idiomatic metaphor lies in the fact that the wheel is the archetype of human ingenuity both by virtue of the added power and flexibility it affords its users and also in the ancient origins which allow it to underlie much if not all of modern technology. As it is not considered to have operational flaws, an attempt to reinvent it would be pointless and add no value to the object, as well as a waste of time diverting the investigator's resources from possibly more worthy goals which his or her skills could advance more substantially.
At the same time, however, reinventing the wheel is an important tool in the instruction of complex ideas. Rather than providing students simply with a list of known facts and techniques and expecting them to incorporate these ideas perfectly and rapidly, the instructor instead will build up the material anew, leaving the student to work out those key steps which embody reasoning characteristic of the field.
The dual meanings of this phrase lend it an ironic flavor when not used derisively, especially when the user applies it reflexively, possibly to indicate that his or her activities might be perceived as merely reinventing the wheel, but that they actually possess additional value.
Related phrases
Not invented here is another phrase that describes a similar attitude that leads to reinventing the wheel. It is commonly used in software development circles where software outputs that many other developers have already done are nevertheless developed locally. In many cases the source code is in the public domain.
Reinventing the square wheel is a variation with even less favourable results.