It is common for the seals between the two panes of glass in a sealed double-glazing unit to a door or window to break down, typically after about ten years. When this happens, condensation forms between the panes. Replacement of the sealed unit (but not always of the frame) is then necessary. You should expect this to happen in due course.
Since April 2002 the replacement of doors and windows has required Building Regulations approval. Alternatively, a contractor registered under the Government’s “competent person scheme” should have installed them. “Competent persons” will be registered by, amongst others, FENSA, SERTASS, DSI and Benchmark.
Your Conveyancer should confirm the replacement windows and doors have a FENSA certificate or its equivalent available as this should ensure they were properly installed.