Suppose an officer arrests you and questions you about your alleged crime. She doesn’t provide the Miranda warning. You confess. Only then does she read you your rights. She begins to question you again. Figuring there’s no point to withholding a confession you’ve already given, you confess again.
The first, Miranda-less statement will probably be inadmissible in court. You do, after all, have a right to be informed of your rights. But what about the second, post-Miranda-warning confession? Will a court allow that one into evidence if you go to trial?
The answer to this question, like so many other legal queries, depends on the facts. In general, though, police officers aren’t allowed to use this kind of “two-step” technique in order to befuddle suspects into confessing. If they use it, both confessions may very well be inadmissible for most purposes.
For much more on this topic, including exceptions, see Mid-Interrogation Miranda Warnings.