mehrfach
repeatedly
Several times; happening repeatedly at different points in time.
Ich habe dich mehrfach angerufen, aber du bist nicht rangegangen.
I called you repeatedly, but you didn't pick up.

repeatedly / multiple
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repeatedly
Several times; happening repeatedly at different points in time.
Ich habe dich mehrfach angerufen, aber du bist nicht rangegangen.
I called you repeatedly, but you didn't pick up.

The word is made up of 'mehr' (more) and the ending '-fach'. The ending '-fach' means '-times' or '-fold' and is also used in other number words such as 'einfach' (single/simple), 'zweifach' (double/twofold), or 'vielfach' (many times/multiple).
Bitte drucken Sie das Dokument in dreifacher Ausführung.
Please print the document in triplicate.
'Mehrfach' can be used as an adverb or an adjective. As an adverb, meaning 'several times', it is not declined. As an adjective, meaning 'present more than once', it comes before a noun and is declined.
Er hat mehrfach versucht, sie zu erreichen.
He tried repeatedly to reach her.
As an adjective, 'mehrfach' (multiple) is often used in connection with titles, awards, or copies. Typical examples are 'mehrfacher Sieger' (multiple winner), 'mehrfache Preisträgerin' (multiple award winner), or 'in mehrfacher Ausfertigung' (in multiple copies).
Der Vertrag muss in mehrfacher Ausfertigung unterschrieben werden.
The contract must be signed in multiple copies.
simple / just
'einfach' and 'mehrfach' can form a contrast when the meaning is 'once' versus 'several times'. 'mehrfach' means that something happens repeatedly or is present in several copies. In certain contexts, 'einfach' can mean the opposite, for example with an 'einfache Fahrt' (one-way trip) without a return journey. Often, however, 'einfach' also means 'easy' or 'not complicated'. This is exactly where the risk of confusion arises: 'mehrfach' is not the opposite of 'easy', but of 'once' or 'only single' in a countable sense.

Vor dem Termin hat die Praxis mehrfach bei mir angerufen.
Before the appointment, the practice called me several times.
Für die kurze Strecke kaufe ich nur eine einfache Fahrkarte.
For the short distance, I only buy a one-way ticket.