Yes, it's not entirely uncommon and can still have a successful outcome depending on how it's handled. If you think they're going to be able to meet the requirements of the job, then manage them closely, but if you know deep down they won't, cut your losses and let them go now. It's shit but it's not a good use of your time or theirs.
Firstly as PPs have said, you need to be absolutely crystal clear with them beyond any doubt about what the expectations of the role are, and how they're falling short of them - give examples where you can. Whatever reasons they might give for not doing the job to the expected standard, you still reiterate what the role requires.
Secondly, you need to set clear parameters for the extension and for what you expect from them in that period. SMART objectives and very clear guidelines, e.g. provide a weekly email summary on a Friday afternoon to confirm that admin tasks have been completed as agreed, etc.
Be absolutely clear that failure to meet all the objectives set for the extended probation will result in failing their probation - they need to know what's at stake and not think that there's another extension available to them at the end of this one. Have regular updates throughout the period to discuss progress and remember you don't have to wait the full 3/6 months etc to make the decision if it's clear they're not going to meet the requirements.
And third - document EVERYTHING. After every meeting send a summary of what was discussed and ask them to respond to confirm or correct any of the details you've provided. Make sure every objective is clearly outlined and agreed to, and you've got a clear understanding of what the consequence of not meeting the required standards will be. It's a lot easier for employees to raise complaints through ACAS now without as much evidence required as previously, and if for any reason they're not happy with the outcome of their probation it is much easier for you to have clear documentation to present back if needed.
On top of these - if this is your first line management role (good luck!) then you should be being closely supported by a more senior manager - don't be afraid to approach them to run them through your plan as they may have insights they can share and will need to support your decision ultimately.